Got around to polishing it a bit, adding a readme and pushing it to codeberg, so there is now codeberg.org/fugi/immich-album-sync :)
This is very much unfinished – I put a roadmap in the readme as an overview of what works (spoilers: only unidirectional sync of simple media, so far) and what doesn't / is untested. Feel free to look, try out, give feedback, send patches, ...