Thanks to all involved! Hope this project has a bright future, already donated.
Hade no GAPPS, made a backup with TWRP to be on the safe side, removed supersu, upgraded dirty all the way (Dalvik / ART cache always cleaned). At a first glance everything what I checked looks good, but surely some time needed to go in deep, some further questions surely will arise ;- )
PS: One thing, proximity sensor isn’t working on the lookscreen. With LineageOS I could swipe over the phone without touching and the lockscreen appeared. IIRC there was a menu point under settings/display to enable/disable this behavior