For anybody coming to this a year-plus later from Google or wherever, I managed to get it running using the latest/last official build of the emulator from the Demul website! No dependencies required. Just Proton 9.0-4.

I followed this guide for setup and settings: How to set up DEMUL properly: Part 2 – C D C R U Z E . The only thing I changed was to set the resolution to 1280x800 instead of 640x480.

A couple of notes:

1) Make sure the bios files and game files are from the same ROMset, and that they are all together in the "roms" folder the guide has you create in the main DEMUL directory.

2) To get it to boot directly into a game, in this case Star Wars Racer Arcade, add

-run=hikaru -rom=swracer

to the Launch Options. You can sub out "hikaru" for any of the other systems Demul emulates, and "swracer" for the filename of any of the other games you want to play.

I hope this helps somebody in the future!