I have figured out the problem.
I tried installing CORE to the M.2 card I have in a USB enclosure.
It seemed to go fine, but when starting TrueNAS CORE it spewed out some message about not being able to access a drive or something (it was on my little 7" monitor I can't remember what it said exactly.)
This is when I tried re-installing CORE to the USB stick I had.
Everything went fine, and it boots up.
So the issue is the M.2 drives in the USB enclosures, for some reason they will not work as boot devices.
So if I can figure out why these won't work as boot devices then i'm sure that'll fix everything. CORE not working properly with them proves it isn't a CORE/SCALE issue but booting from these drives issue.
How exactly should they be formatted / partitioned / etc? Like I said I tried making them GPT with a fat32 filesystem (I know the filesystem is wiped anyway upon install but just did it for compatability purposes instead of leaving them RAW) but that didn't seem to do it. Do they need to be MBR? should I set any 'flags' on them using GParted like maybe 'bootable'?
From one of those threads linked:
"I tried a M.2 NVME to USB-C adapter and it also was not booting. Such a stupid behaviour from SCALE here."
So it seems it's definitely an M2 to usb adapter issue. This sucks because I have 0 way of knowing which enclosure will or will not work now, and I don't want to buy every £5-20 adapter/enclosure I can find until one works.
I've been meaning to 'upgrade' from using an SD card in a USB adapter for my boot device for almost a year and I finally get around to it and decide to finally migrate to core at the same time and I just can't.