So my first step was a backup of data!
As far as I know there is no "default" slot. I think if you flash a rom, the device will recognise it and flash it to "the other" slot. That's the reason I switched to B before flashing the update. After that you need to flash the recovery, because it's part of the rom and will be stock lineage after a reboot.
After flashing TWRP you can reboot and your active slot is now "the other one". I switched to B before the update, so my updated slot A is now active. Now you can flash Magisk and GApps.
I didn't flashes GApps the first time and lost them, so it seems to be necessary to flash them after every update. Restored my data after the first few tries. Now it works perfect.
Thanks for the reply! It really helped and I upgraded without issues.