I have flashed twrp 3.5.0 and it works fine - so I guess it is possible.Thanks for your reply!
In the meantime i managed to install Android 9.0 as Stock rom.
On 8.0 I always soft bricked my device when trying to install TWRP via fastboot. Do you think it is now possible to install TWRP without this issue?
If the ROM firmware of Android 9 is required for this custom rom to work, is it "fully" android 11 then? In other words, is the base still android 9?
Question about system architecture should be directed to it's developer. For me rom looks great and behaves like 11 - so everything is like it should be Stock 9 is the very last released by Motorola for this device, so somehow it has to be the base for all successors.