The EA App makes this difficult to launch because it pops up two empty screens. Before attempting to run the game, be sure your Steam ID is associated with your account on EA.com. You must have a network connection each time you launch the game or you could end up in an unlaunchable state (if this occurs, delete proton files from Developer settings to start from scratch).

For first run, click the button to run the EA app installer. Wait for up to two blank screens, and if the game doesn't load, exit and re-enter. If your Steam ID has been associated with your EA account, it should launch after that first time (and possibly the first time).

Subsequent runs should be fine provided you have an Internet connection when you launch the game. You should see two blank screens that auto-dismiss after several seconds before the actual game launches.

If the game ever ends up in an unlaunchable state, use the game's Developer settings to delete Proton files.

Audio sometimes will crackle for some time after waking the Steam Deck if it was put to sleep without pausing first. Usually the problem will self-correct.

I've replayed the end of Threedom several times now (even with the boss fight after), and on multple reloads, I end up back at the same spot having lost game progress.