Investigate what's missing with EasyAntiCheat
Not sure what's missing here, it's a known unknown. Collate a list of games that use EAC and have the Proton support enabled, then try to find a game that is relatively small for testing.
Apparently Fall Guys was only like 8GB, but it recently updated and removed support for Proton.
Here's a list of game using EAC from steamdb (afaik, it should be every game) https://steamdb.info/instantsearch/?refinementList%5Btechnologies%5D%5B0%5D=AntiCheat.EasyAntiCheat
And here's the list of game that have linux support enabled https://steamdb.info/search/?a=app_keynames&type=-1&keyname=528&operator=1&keyvalue=1826330 (1826330 is the proton eac runtime app id)
STAR WARS™: Squadrons could be a good fit for your tests
VRChat is a ~1GB initial download (not counting worlds/avatars etc), is free to play, and has a desktop mode that doesn't require a VR headset.
Ah yeah indeed, it supports easy anticheat on linux but doesn't have the easy anticheat runtime as a requirement (unaware if that's something that devs have to set or they have to ask steam (dev) support for it)
That would appear to be because it does not, in fact, require the Proton EasyAntiCheat Runtime, having just gone and checked on my x86_64 box. I'm going to refrain from speculating on why. I do know that EAC is running, though, because the linux VR community occasionally gets spurious random EAC failures and has startup scripts to try to mitigate that.
(i have no idea if this is on-topic or not at this point tbh)