It dit not work to downgrade to Android 9. I didn't get Magisk to run. But LineageOS worked and that is Android 9. everything is working fine so far, need some time to check the battery but I know that Android 9 was much better for my battery before I upgraded to Android 10. And I think there are no features which are missing now. New is not always better.
If you want to use Lineage, OpenGapps did not work because of the system partition size, I had to use MindTheGapps. Flashing Magisk was no problem but I don't like the Recovery from Lineage, so I flashed twrp after install.