Ok, I manually added these options to the octavia.conf file (actually I forked the charm and changed the template):

[controller_worker]
<...>
# defaults are: size=16GB, type=None, create_retry_interval=5, create_timeout=300, create_max_retries=5
volume_driver = volume_cinder_driver
volume_size = 16
volume_type = usc01-az01-abs-octavia
volume_create_retry_interval = 5
volume_create_timeout = 300
volume_create_max_retries = 5

The usc01-az01-abs-octavia type exists:

ubuntu@jumphost:~/2021-04-12-TCS-Prod1PCB-215965$ openstack volume type list
+--------------------------------------+------------------------+-----------+
| ID                                   | Name                   | Is Public |
+--------------------------------------+------------------------+-----------+
| 884217ee-d402-4644-b421-931517025172 | usc01-az01-abs-octavia | True      |
| 6af193ca-970d-4b78-9b0d-b90bbceced93 | usc01-az01-abs-stable1 | True      |
| a45fe4b1-ac8b-40d8-b3c1-45b00c64b075 | usc01-az01-abs-stable2 | True      |
| 5e44595b-012c-41b3-a842-b8c1637a4000 | usc01-az01-abs-arbor   | True      |
+--------------------------------------+------------------------+-----------+

ubuntu@jumphost:~/2021-04-12-TCS-Prod1PCB-215965$ openstack volume type show usc01-az01-abs-octavia
+--------------------+------------------------------------------------------------------------------------------+
| Field              | Value                                                                                    |
+--------------------+------------------------------------------------------------------------------------------+
| access_project_ids | None                                                                                     |
| description        | None                                                                                     |
| id                 | 884217ee-d402-4644-b421-931517025172                                                     |
| is_public          | True                                                                                     |
| name               | usc01-az01-abs-octavia                                                                   |
| properties         | RESKEY:availability_zones='us-central-1-az01', volume_backend_name='cinder-ceph-stable2' |
| qos_specs_id       | None                                                                                     |
+--------------------+------------------------------------------------------------------------------------------+

But when trying to create loadbalancers, I get this error:

2021-05-25 17:29:16.550 3666784 ERROR oslo_messaging.rpc.server taskflow.exceptions.WrappedFailure: WrappedFailure: [Failure: octavia.common.exceptions.ComputeBuildException: Failed to build compute instance due to: Multiattach volumes are only supported starting with compute API version 2.60. (HTTP 400) (Request-ID: req-16ce1668-84f6-4e88-84db-724cc3bc1538), Failure: octavia.common.exceptions.ComputeBuildException: Failed to build compute instance due to: Multiattach volumes are only supported starting with compute API version 2.60. (HTTP 400) (Request-ID: req-bdae919c-19c2-4ff5-b770-00a8d54a5f18)]

As you can see the volume type does not have multiattach property set (but, just FYI the other three volume types do have it enabled).

Any ideas of what might be wrong?