I’d imagine mpd with one of many frontends would work well enough. You’d just need to use a dummy music library directory with symlinks to your four music storages for mpd to pick up and catalog everything.

Tbf to cloud sync, nothing is stopping you from using your own backup/restore service with your drm-free titles compared to the other features that Galaxy offers.

GOG has DRM for many titles: see Galaxy. As I understand it, it isn’t as pervasive as Steam, but is necessary if you want multiplayer on many titles or care about extras like achievements.