Burned a few hours messing around further, unfortunately no progress was made. A few further notes:
Enabling above 4g encoding caused a black screen, and maybe a post failure? not 100% sure since I couldn’t see anything
UEFI Only didn’t really work, the truenas boot drive didn’t show up at all after doing that, and even trying to boot to the UEFI version of the installer usb boot device (it shows up twice, one of which has UEFI preceding it) fails to a grub rescue prompt (unknown file system).
tried changing the UEFI mode in Rufus when making the installer USB, tried both options during the installer when asked if it should support EFI. also tried different SATA ports, different SATA cable… at this point I’m feeling out of options. Might be a sign from above that it’s time to upgrade 