A lot of selfhosted tools use metadata from open-source providers.

So if something is missing or incorrect consider updating it, for example:

Lidarr -> MusicBrainz
Sonarr -> TVDB
Radarr -> TMDB
Readarr -> ????

Also I recently discovered Hardcover.app and I think this will be an excellent replacement for Goodreads.
And I think it will be open source in the near future, if I read that correctly.

And that you wished you knew from the start?

Mine is: - Custom Formats in Radarr/Sonarr