Finally, I’m able to give an update. In summary it is working!
First I did was try a beta release of Ropieee from a couple of weeks ago. I couldnt get that to boot up at all, so I moved on to update the firmware of the Arche. It took a couple of weeks to track down the firmware via Focal and the local importer and also the upgrade was different from what was written in the Arche manual. I had to find a Windows PC too as the upgrade can only be done from Windows.
So after installing the firmware finally today, I also had to re-flash my Ropieee as the beta didn’t work. Turns out a new stable version was released on December 5 and I used that.
I can confirm the combination of the new firmware and the new Ropieee release works and I can now play from Roon to the Arche. Unfortunately I don’t know if it was actually the firmware upgrade or the new Ropieee version that did the trick.
Thanks to all who have responded to this thread!