I’m also looking for something like this. Personally, I do use RetroArch pointing to a network share, too. However I’ve really liked using OpenEmu on Mac. Not what you’re looking for (also not self-hosted), but I think it’s one of the nicest UI I’ve seen for organizing and playing roms.
I typically either use Audionews or AudioZ. That has most things I’m looking for. Occasionally some random/obscure VSTs pops up in Rutracker. I use Kontakt for everything so keep in mind that it can be tricky to get libraries added to Kontakt these days depending on the version and OS you’re running.