I've verified almost all the other components now, hdmi, scart, vga, analogue audio out, ethernet, fan headers, switches, everything seems to work. As people have said the USB part of the board seems to be isolated from the rest. The HDMI and Ethernet also seem to be isolated pass-through connections.
Don't have anything on hand to test the optical digital audio, but the port is lit up so I'm guessing it works. The PSX SNAC adapter didn't work despite toggling to 3.3V mode. After researching this I think this is due to the multisystem not being compatible which is a bit annoying tbh as this was the main controller I wanted to use with SNAC.
Also assembled the case. I was considering getting it 3D printed from elsewhere but the prices elsewhere seemed more expensive than the official store, which makes sense considering bulk vs one-off.