Thank you @HoneyBadger, we have a solution!
I booted up a Fedora live CD and wiped both my boot drives with zeros, took about an hour. I also wrote 100MB of zeros to the beginning of every other drive, being impatient.
Installed TrueNas with no swap and everything is working. Boots up no problems, setup pools, all disks report healthy and so on. I believe the root cause of this problem was unhelpful 1s and 0s on the boot drives that the installer wasn’t wiping. Having a swap partition meant that the unhelpful 1s and 0s didn’t cause an issue. What an annoying bug.
1 week later and an unhealthy collection of USB/SATA adaptors, I’ll carry on setting up my TrueNAS server. lol.
Finally, I do still get these errors coming out just before the GRUB screen. I believe they are unrelated to my boot problem. There are quite a few threads about similar messages that just blame GRUB and say not to worry about it. Do you think it’s safe to ignore these?
Thank you so much for your help. Easily one of the nicest open source forums I’ve been on.