Took a while to even get OPNsense running because I couldn't remember which USB stick was which in BIOS, lol. One of them actually says PMAP; quite confusing, really.

The migration to OPNsense 23.1 was a success. The firewall now uses an SSD that is GPT/EFI partitioned instead of an HDD that is MBR/BIOS partitioned.

Now I can use the HDD for something else, finally.