Hi @Steckelfisch, welcome to the e forum. The late response is due to your question being posted in the wrong place really. I was waiting to see if admin would move it somehow, but as your post is technically a “reply” it might not be so easy!
Anyway, I will reply to your question but I will probably remove it after a few days, or when I know you have seen it, just to keep this space free from off topic stuff.
Don’t know the tech answer, but it’s generally advised against downgrading OS versions. At the moment the ‘dev’ version of e is oreo android 8 and as you say, you have pie android 9 on your device.
So you would be better to try installing, for example, the pie test build which works very well. This of course is assuming you have not in the interim installed android 10 based rom on your phone. If you have, it would be better to wait for the q android 10 release for this device which will be arriving next week with a bit of luck.