Can't boot to system (lineageOS) after updating TWRP
Solved it without any data loss by putting a zip of the same LineageOS version I was using on a SD card and re-flashing with that from TWRP 3.3.1 (wiping only cache and Dalvik).

Thanks @Robert.

I still don't understand how exactly I screwed my system partition in the first place but I'm happy anyway.