I do own RPG Maker MV, so I'll go ahead and give that a try next time I have some spare time and see how it goes. For what it's worth though other things that used to work are breaking in a similar fashion right now, so I think the most recent SteamOS update just broke some compatibility thing, which wouldn't make it a problem with the Demake project.
Viewing post in Dissidia Final Fantasy Opera Omnia: Demake Edition comments
Thats a bunch of factors i think that could cause problems here. Im unsure if Linux would need .ogg or .mp4 for audio, so idk if music or audio would work. (I only imported .ogg to save space). If linux or steam encrypts ant of the files, then it may have problems understanding how to open files that out of the rpg norm, cause I use 4-5 different folders to space images. But once I get season 2 stable, I dont mind providing you project file to try out on the steam deck. Id interested to get it working on the deck. Would actually love it. But season 2 won't be out until beginning or September or late August. If I run into delays, then im going to wait until after Final Fantasy Resonance has been out for awhile (late November or December). So if you are interested in playing it, id recommend following the page, so you can get update announcement. Season 2 is a big update, so its taking a bit.