im game mode i have the same problem. you can switch to desktop, there it works and then in game mode you are logged in already

or you could set the mouse on one of the touch pads and mouse clicks on the back pedals. then you do not need to switch the profile

i use manjaro for about 3 years as my daily driver and do not have problems at all.

you could give emuze a try: https://github.com/bmsuseluda/emuze

i worked on it for about 2 years and released it a few days a go. it is a passion project of mine. maybe you like it 🙃

as a workaround i put the steam deck in the dock without puting the tv on. with this the steam deck can download with display off.