Looks like I could archive about 1.3TB, of a 1.64TB snapshot, before things started to go bad, again:

[110079.681102] INFO: task z_wr_iss_h:2171 blocked for more than 120 seconds.
[110079.688123]       Tainted: P         C OE     5.4.0-1018-raspi #20-Ubuntu
[110079.695167] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[110079.703260] z_wr_iss_h      D    0  2171      2 0x00000028
[110079.703276] Call trace:
[110079.703297]  __switch_to+0x104/0x170
[110079.703308]  __schedule+0x30c/0x7c0
[110079.703317]  schedule+0x3c/0xb8
[110079.703325]  io_schedule+0x20/0x58
[110079.703338]  rq_qos_wait+0x100/0x178
[110079.703347]  wbt_wait+0xb4/0xf0
[110079.703357]  __rq_qos_throttle+0x38/0x50
[110079.703370]  blk_mq_make_request+0x128/0x610
[110079.703382]  generic_make_request+0xb4/0x2d8
[110079.703392]  submit_bio+0x48/0x218
[110079.703629]  vdev_disk_io_start+0x670/0x9f8 [zfs]
[110079.703850]  zio_vdev_io_start+0xdc/0x2b8 [zfs]
[110079.704068]  zio_nowait+0xd4/0x170 [zfs]
[110079.704285]  vdev_mirror_io_start+0xa8/0x1b0 [zfs]
[110079.704503]  zio_vdev_io_start+0x248/0x2b8 [zfs]
[110079.704720]  zio_execute+0xac/0x110 [zfs]
[110079.704760]  taskq_thread+0x2f8/0x570 [spl]
[110079.704773]  kthread+0xfc/0x128
[110079.704784]  ret_from_fork+0x10/0x1c
[110079.704795] INFO: task z_wr_int:2173 blocked for more than 120 seconds.
[110079.711650]       Tainted: P         C OE     5.4.0-1018-raspi #20-Ubuntu
[110079.718668] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[110079.726745] z_wr_int        D    0  2173      2 0x00000028
[110079.726760] Call trace:
[110079.726780]  __switch_to+0x104/0x170
[110079.726790]  __schedule+0x30c/0x7c0
[110079.726803]  schedule+0x3c/0xb8
[110079.726811]  io_schedule+0x20/0x58
[110079.726823]  rq_qos_wait+0x100/0x178
[110079.726832]  wbt_wait+0xb4/0xf0
[110079.726842]  __rq_qos_throttle+0x38/0x50
[110079.726854]  blk_mq_make_request+0x128/0x610
[110079.726866]  generic_make_request+0xb4/0x2d8
[110079.726877]  submit_bio+0x48/0x218
[110079.727100]  vdev_disk_io_start+0x670/0x9f8 [zfs]
[110079.727320]  zio_vdev_io_start+0xdc/0x2b8 [zfs]
[110079.727549]  zio_nowait+0xd4/0x170 [zfs]
[110079.727766]  vdev_queue_io_done+0x1ec/0x2a0 [zfs]
[110079.727991]  zio_vdev_io_done+0xec/0x220 [zfs]
[110079.728209]  zio_execute+0xac/0x110 [zfs]
[110079.728255]  taskq_thread+0x2f8/0x570 [spl]
[110079.728268]  kthread+0xfc/0x128
[110079.728279]  ret_from_fork+0x10/0x1c
[110079.728309] INFO: task txg_sync:2388 blocked for more than 120 seconds.
[110079.735157]       Tainted: P         C OE     5.4.0-1018-raspi #20-Ubuntu
[110079.742175] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[110079.750306] txg_sync        D    0  2388      2 0x00000028
[110079.750321] Call trace:
[110079.750337]  __switch_to+0x104/0x170
[110079.750348]  __schedule+0x30c/0x7c0
[110079.750356]  schedule+0x3c/0xb8
[110079.750366]  schedule_timeout+0x9c/0x190
[110079.750374]  io_schedule_timeout+0x28/0x48
[110079.750412]  __cv_timedwait_common+0x1a8/0x1f8 [spl]
[110079.750447]  __cv_timedwait_io+0x3c/0x50 [spl]
[110079.750670]  zio_wait+0x130/0x2a0 [zfs]
[110079.750890]  dsl_pool_sync+0x3fc/0x498 [zfs]
[110079.751109]  spa_sync+0x538/0xe68 [zfs]
[110079.751326]  txg_sync_thread+0x2c0/0x468 [zfs]
[110079.751364]  thread_generic_wrapper+0x74/0xa0 [spl]
[110079.751377]  kthread+0xfc/0x128
[110079.751388]  ret_from_fork+0x10/0x1c
[110079.751459] INFO: task borg:1063669 blocked for more than 120 seconds.
[110079.758224]       Tainted: P         C OE     5.4.0-1018-raspi #20-Ubuntu
[110079.765237] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[110079.773310] borg            D    0 1063669 1063661 0x00000000
[110079.773324] Call trace:
[110079.773344]  __switch_to+0x104/0x170
[110079.773355]  __schedule+0x30c/0x7c0
[110079.773366]  schedule+0x3c/0xb8
[110079.773406]  cv_wait_common+0x188/0x1b0 [spl]
[110079.773441]  __cv_wait+0x30/0x40 [spl]
[110079.773664]  zil_commit_impl+0x234/0xd30 [zfs]
[110079.773882]  zil_commit+0x48/0x70 [zfs]
[110079.774112]  zfs_write+0xa3c/0xb90 [zfs]
[110079.774331]  zpl_write_common_iovec+0xac/0x120 [zfs]
[110079.774557]  zpl_iter_write+0xe4/0x150 [zfs]
[110079.774569]  new_sync_write+0x100/0x1a8
[110079.774578]  __vfs_write+0x74/0x90
[110079.774593]  vfs_write+0xe4/0x1c8
[110079.774603]  ksys_write+0x78/0x100
[110079.774612]  __arm64_sys_write+0x24/0x30
[110079.774622]  el0_svc_common.constprop.0+0x80/0x218
[110079.774631]  el0_svc_handler+0x34/0xa0
[110079.774640]  el0_svc+0x10/0x2cc
[110079.774665] INFO: task z_wr_iss_h:3853507 blocked for more than 120 seconds.
[110079.781953]       Tainted: P         C OE     5.4.0-1018-raspi #20-Ubuntu
[110079.788973] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[110079.797052] z_wr_iss_h      D    0 3853507      2 0x00000028
[110079.797067] Call trace:
[110079.797083]  __switch_to+0x104/0x170
[110079.797094]  __schedule+0x30c/0x7c0
[110079.797102]  schedule+0x3c/0xb8
[110079.797110]  io_schedule+0x20/0x58
[110079.797123]  rq_qos_wait+0x100/0x178
[110079.797132]  wbt_wait+0xb4/0xf0
[110079.797142]  __rq_qos_throttle+0x38/0x50
[110079.797154]  blk_mq_make_request+0x128/0x610
[110079.797165]  generic_make_request+0xb4/0x2d8
[110079.797176]  submit_bio+0x48/0x218
[110079.797398]  vdev_disk_io_start+0x670/0x9f8 [zfs]
[110079.797616]  zio_vdev_io_start+0xdc/0x2b8 [zfs]
[110079.797834]  zio_nowait+0xd4/0x170 [zfs]
[110079.798050]  vdev_mirror_io_start+0xa8/0x1b0 [zfs]
[110079.798267]  zio_vdev_io_start+0x248/0x2b8 [zfs]
[110079.798484]  zio_execute+0xac/0x110 [zfs]
[110079.798522]  taskq_thread+0x2f8/0x570 [spl]
[110079.798535]  kthread+0xfc/0x128
[110079.798545]  ret_from_fork+0x10/0x1c
[110079.798674] INFO: task z_wr_iss_h:3853521 blocked for more than 120 seconds.
[110079.805950]       Tainted: P         C OE     5.4.0-1018-raspi #20-Ubuntu
[110079.812940] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[110079.820986] z_wr_iss_h      D    0 3853521      2 0x00000028
[110079.820998] Call trace:
[110079.821007]  __switch_to+0x104/0x170
[110079.821014]  __schedule+0x30c/0x7c0
[110079.821017]  schedule+0x3c/0xb8
[110079.821020]  io_schedule+0x20/0x58
[110079.821026]  rq_qos_wait+0x100/0x178
[110079.821031]  wbt_wait+0xb4/0xf0
[110079.821035]  __rq_qos_throttle+0x38/0x50
[110079.821040]  blk_mq_make_request+0x128/0x610
[110079.821045]  generic_make_request+0xb4/0x2d8
[110079.821050]  submit_bio+0x48/0x218
[110079.821157]  vdev_disk_io_start+0x670/0x9f8 [zfs]
[110079.821248]  zio_vdev_io_start+0xdc/0x2b8 [zfs]
[110079.821343]  zio_nowait+0xd4/0x170 [zfs]
[110079.821430]  vdev_mirror_io_start+0xa8/0x1b0 [zfs]
[110079.821521]  zio_vdev_io_start+0x248/0x2b8 [zfs]
[110079.821608]  zio_execute+0xac/0x110 [zfs]
[110079.821627]  taskq_thread+0x2f8/0x570 [spl]
[110079.821633]  kthread+0xfc/0x128
[110079.821638]  ret_from_fork+0x10/0x1c
[110079.821643] INFO: task z_wr_iss_h:3853701 blocked for more than 120 seconds.
[110079.828906]       Tainted: P         C OE     5.4.0-1018-raspi #20-Ubuntu
[110079.835893] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[110079.843938] z_wr_iss_h      D    0 3853701      2 0x00000028
[110079.843944] Call trace:
[110079.843951]  __switch_to+0x104/0x170
[110079.843956]  __schedule+0x30c/0x7c0
[110079.843959]  schedule+0x3c/0xb8
[110079.843962]  io_schedule+0x20/0x58
[110079.843967]  rq_qos_wait+0x100/0x178
[110079.843971]  wbt_wait+0xb4/0xf0
[110079.843975]  __rq_qos_throttle+0x38/0x50
[110079.843980]  blk_mq_make_request+0x128/0x610
[110079.843985]  generic_make_request+0xb4/0x2d8
[110079.843989]  submit_bio+0x48/0x218
[110079.844083]  vdev_disk_io_start+0x670/0x9f8 [zfs]
[110079.844170]  zio_vdev_io_start+0xdc/0x2b8 [zfs]
[110079.844257]  zio_nowait+0xd4/0x170 [zfs]
[110079.844343]  vdev_mirror_io_start+0xa8/0x1b0 [zfs]
[110079.844430]  zio_vdev_io_start+0x248/0x2b8 [zfs]
[110079.844517]  zio_execute+0xac/0x110 [zfs]
[110079.844533]  taskq_thread+0x2f8/0x570 [spl]
[110079.844539]  kthread+0xfc/0x128
[110079.844543]  ret_from_fork+0x10/0x1c
[110079.844547] INFO: task z_wr_iss_h:3853747 blocked for more than 120 seconds.
[110079.851801]       Tainted: P         C OE     5.4.0-1018-raspi #20-Ubuntu
[110079.858786] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[110079.866829] z_wr_iss_h      D    0 3853747      2 0x00000028
[110079.866835] Call trace:
[110079.866844]  __switch_to+0x104/0x170
[110079.866849]  __schedule+0x30c/0x7c0
[110079.866853]  schedule+0x3c/0xb8
[110079.866857]  io_schedule+0x20/0x58
[110079.866862]  rq_qos_wait+0x100/0x178
[110079.866866]  wbt_wait+0xb4/0xf0
[110079.866870]  __rq_qos_throttle+0x38/0x50
[110079.866875]  blk_mq_make_request+0x128/0x610
[110079.866880]  generic_make_request+0xb4/0x2d8
[110079.866884]  submit_bio+0x48/0x218
[110079.866980]  vdev_disk_io_start+0x670/0x9f8 [zfs]
[110079.867071]  zio_vdev_io_start+0xdc/0x2b8 [zfs]
[110079.867166]  zio_nowait+0xd4/0x170 [zfs]
[110079.867253]  vdev_mirror_io_start+0xa8/0x1b0 [zfs]
[110079.867344]  zio_vdev_io_start+0x248/0x2b8 [zfs]
[110079.867431]  zio_execute+0xac/0x110 [zfs]
[110079.867451]  taskq_thread+0x2f8/0x570 [spl]
[110079.867457]  kthread+0xfc/0x128
[110079.867462]  ret_from_fork+0x10/0x1c
[110079.867466] INFO: task z_wr_int:3853774 blocked for more than 121 seconds.
[110079.874546]       Tainted: P         C OE     5.4.0-1018-raspi #20-Ubuntu
[110079.881532] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[110079.889574] z_wr_int        D    0 3853774      2 0x00000028
[110079.889580] Call trace:
[110079.889588]  __switch_to+0x104/0x170
[110079.889592]  __schedule+0x30c/0x7c0
[110079.889595]  schedule+0x3c/0xb8
[110079.889599]  io_schedule+0x20/0x58
[110079.889604]  rq_qos_wait+0x100/0x178
[110079.889607]  wbt_wait+0xb4/0xf0
[110079.889611]  __rq_qos_throttle+0x38/0x50
[110079.889616]  blk_mq_make_request+0x128/0x610
[110079.889621]  generic_make_request+0xb4/0x2d8
[110079.889625]  submit_bio+0x48/0x218
[110079.889721]  vdev_disk_io_start+0x670/0x9f8 [zfs]
[110079.889809]  zio_vdev_io_start+0xdc/0x2b8 [zfs]
[110079.889896]  zio_nowait+0xd4/0x170 [zfs]
[110079.889982]  vdev_queue_io_done+0x1ec/0x2a0 [zfs]
[110079.890069]  zio_vdev_io_done+0xec/0x220 [zfs]
[110079.890156]  zio_execute+0xac/0x110 [zfs]
[110079.890172]  taskq_thread+0x2f8/0x570 [spl]
[110079.890177]  kthread+0xfc/0x128
[110079.890181]  ret_from_fork+0x10/0x1c
[110200.513965] INFO: task z_wr_iss_h:2171 blocked for more than 241 seconds.
[110200.520988]       Tainted: P         C OE     5.4.0-1018-raspi #20-Ubuntu
[110200.528031] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[110200.536134] z_wr_iss_h      D    0  2171      2 0x00000028
[110200.536150] Call trace:
[110200.536170]  __switch_to+0x104/0x170
[110200.536182]  __schedule+0x30c/0x7c0
[110200.536191]  schedule+0x3c/0xb8
[110200.536199]  io_schedule+0x20/0x58
[110200.536212]  rq_qos_wait+0x100/0x178
[110200.536222]  wbt_wait+0xb4/0xf0
[110200.536232]  __rq_qos_throttle+0x38/0x50
[110200.536244]  blk_mq_make_request+0x128/0x610
[110200.536256]  generic_make_request+0xb4/0x2d8
[110200.536266]  submit_bio+0x48/0x218
[110200.536505]  vdev_disk_io_start+0x670/0x9f8 [zfs]
[110200.536726]  zio_vdev_io_start+0xdc/0x2b8 [zfs]
[110200.536945]  zio_nowait+0xd4/0x170 [zfs]
[110200.537161]  vdev_mirror_io_start+0xa8/0x1b0 [zfs]
[110200.537380]  zio_vdev_io_start+0x248/0x2b8 [zfs]
[110200.537597]  zio_execute+0xac/0x110 [zfs]
[110200.537638]  taskq_thread+0x2f8/0x570 [spl]
[110200.537651]  kthread+0xfc/0x128
[110200.537662]  ret_from_fork+0x10/0x1c