Well there’s only /etc/fstab (no mstab, vfstab, etc) and it doesn’t have anything other than the boot volume. Systemd mounts don’t seem to be initiating anything related. It appears to be something IX systems or ZFS is doing and the services I found for mounting ZFS with systemd weren’t enabled.
I’ve decided as a stop-gap solution to just boot it up, reinstall TrueNAS, import the pools, and hope I don’t have to reboot until I can save up for a replacement system. If anyone has some good ideas I’m open to giving them a shot. Thank you to everyone who’s helped and/or suggested things to try.
As for the replacement, I’m looking at a TrueNAS Mini R and loading it up with 6 20TB drives in a single RAIDz2. At a later time I can throw in another 6 drives to increase total capacity to 160TB. I was leaning towards a Synology RS2421+ largely due to the flexibility of adding drives as required, but their recent restrictions on 3rd party hard drives makes it more expensive than the Mini R.
This current system was designed at a time when it was the primary SAN/NAS for my homelab and so performance for VMs and similar workloads was paramount. It has since become a media server along with backups with the homelab stuff moving to a Turing Pi v2 due to power consumption, better resiliency, and quieter operation.
Update: I completely reinstalled TrueNAS SCALE. Pools imported just fine, but learned SCALE doesn’t support multipathing yet so I shut it down to remove the other HBA. Booted back up without errors. No idea if it was a lucky boot, some configuration issue (which wouldn’t make sense with the issues I had with Ubuntu), something related to the shares or services relying on the pools (same thing as above), or something else entirely.
Once I have something else to transfer all the data off to I may try a few more things, but things appear to be running well for now.