I have to send in my Deck for an RMA. As I had to reimage it anyways I decided to play around with OpenSUSE Tumbleweed. It took some small trial and error, but by booting into the Gnome live image I could enable the on screen keyboard in the accessibility settings and thus complete the install of Tumbleweed without any additional problems.
I was especially surprised to find that Gnome would turn the screen around correctly by itself. With KDE Plasma I had to set the correct screen orientation myself. And unfortunately Plasma also did not come with any on screen keyboard so it was effectively unusable.
But if you like Gnome you can put it onto your Deck without any additional hardware besides the SD card or USB stick. From a first look everything was working like it was supposed to.
I know what I will be doing as soon as I get my Deck back.
Game mode is Big Picture running on a gamescope-only session.
You can get that on other distros, like ChimeraOS does [0] but it takes a bit of work and some options will probably not work the way you expect them.
Particularly anything to do with Networking config or Bluetooth will probably not be present at all.
[0] https://github.com/ChimeraOS/gamescope-session