I added the Ubisoft Connect installer as a non Steam game, installed it with GE-Proton9-4, and then switched the shortcut to the location of "UbisoftConnect.exe".
I have at least successfully started it and played two matches unranked.
I run it through Lutris (Start Ubisoft connect from Lutris) and the start xdefiant through Ubisoft connect.
I followed some Indian guys tutorial on YouTube.
It crashed two times and trying to start it a third time, Lutris gave an error sth was missing (don't remember what exactly) and offered to install it.
After that it worked but probably more coincidence than knowing what I did.
The tutorial added two game execution properties and installed some windows dll through winetricks.
I got it to work perfectly from this guide: https://piped.video/watch?v=_PLEm1v9bzw Works really well high fps and all. Something I dont think the guide mentions is updating the battle eye runtime using the one from steam.
Tried a couple times - installing via lutris, or proton, played with some winetricks etc. Eventually the launcher worked, then more tweaks for the game to load. But then gave up once it crashed to desktop loading the game every time.