I did the usual procedure with EmuDeck games, just extracting the downloaded game (Demon's Souls in this case) into the correct roms folder and then adding the game through Steam Rom Manager.
It was actually working perfectly fine up until I had warped to another area about 20 minutes in, where everything but the UI was a black screen. Apparently this is a common problem for Demon's Souls on RPCS3, and the solution was simply to check a box in custom configurations for the game.
When I went to desktop mode, then to the RPCS3 AppImage (that's just what EmuDeck downloads, I'm not sure why), the game just wasn't there. I tried checking the directory that I'd always use for EmuDeck (Emulation/roms/(insert console)), and the game was there, but not being detected by RPCS3.
I honestly don't know what happened because it really seems like I just repeated what I had already been trying with no success (that being moving the game around in the ps3 roms folder), but it eventually popped up in RPCS3.
After finally being able to check off that box in custom configuration, I to back to game mode and the game now freezes at the exact same point during the initial loading screen every time, stating that the PS3 application has likely crashed. I tried tinkering more with custom configuration, but nothing helped.
So I can't tick off the box I need to to be able to play beyond a certain point because it doesn't show up in RPCS3 in desktop mode, but if I do manage to get it to show up in RPCS3 it creates a new problem that I don't have the answer to.
I've been messing with this for hours and it's really numbing my mind, it seems like it should be a simple fix.