Comment 6 for bug 1843222

The upgrade failed, in your logs, you have:
"/etc/kernel/postinst.d/initramfs-tools:
update-initramfs: Generating /boot/initrd.img-5.2.0-15-generic
I: The initramfs will attempt to resume from /dev/sda2
I: (UUID=bf02ddd4-8d65-40d6-ab24-4fc8a5673dc6)
I: Set the RESUME variable to override this.
/etc/kernel/postinst.d/zz-update-grub:
Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/init-select.cfg'
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-5.2.0-15-generic
Found initrd image: /boot/initrd.img-5.2.0-15-generic
Found linux image: /boot/vmlinuz-5.0.0-27-generic
Found initrd image: /boot/initrd.img-5.0.0-27-generic
Found linux image: /boot/vmlinuz-5.0.0-25-generic
Found initrd image: /boot/initrd.img-5.0.0-25-generic
cannot open 'This': no such pool
run-parts: /etc/kernel/postinst.d/zz-update-grub exited with return code 1
dpkg: error processing package linux-image-5.2.0-15-generic (--configure):
 installed linux-image-5.2.0-15-generic package post-installation script subprocess returned error exit status 1
Processing triggers for dbus (1.12.14-1ubuntu2) ...
Processing triggers for initramfs-tools (0.133ubuntu10) ...
update-initramfs: Generating /boot/initrd.img-5.2.0-15-generic
I: The initramfs will attempt to resume from /dev/sda2
I: (UUID=bf02ddd4-8d65-40d6-ab24-4fc8a5673dc6)
I: Set the RESUME variable to override this.
Processing triggers for libgdk-pixbuf2.0-0:amd64 (2.39.2-3) ...
Processing triggers for rygel (0.38.1-2ubuntu2) ...
Errors were encountered while processing:
 zfsutils-linux
 zfs-initramfs
 friendly-recovery
 zfs-zed
 grub-pc
 linux-image-5.2.0-15-generic
Log ended: 2019-09-09 01:10:12"

See the "cannot open 'This': no such pool" while updating grub. This is why you have kernel 5.0 after it and don't have latest zfs after upgrade.

You mentionned multiple times a "custom" grub script, does it have "This" somewhere? We don't have any "This" in the distribution script.