Hello again, I can confirm that dual boot with Ubuntu Touch and current LineageOS 19.1 is working.
Some small things have changed over the year:
Magisk (needed to get root access in LineageOS) can not be used as version 21.4 (last version available as zip) in Android12 (it does not turn up as installed app), so sideloading that doesn't make sense.
Instead the way with patching boot.img is needed. But it turns out to be quite easy:
The file "boot.img" has not to be extracted from full nightly build img, the file is exactly the same as the recovery-pro1.img .
Just download linea