so, managed to bring new life in my beloved device.
1. disassemled the unit to accss and manually charged the battery, this stopped the strange boot loop.
(phone still only goes to fastboot)
2. instead of using my pretty new computer running win10 I connected it to my 13 year old laptop running Debian made it able to communicate in fastboot.
3. flashed original ROM via fastboot
(phone still only goes to fastboot)
4. (almost a bit embarresed to say but...) manually change active slot via fastboot. Now it boots 😁
5. flash Lineage again and change active