REAPER is absolutely one of the best pieces of software out there. I've been using it too since maybe 2009, though not so much in the last few years (not moved to an alternative, I'm just not doing so much audio these days).
I love the business model, the development cycle, etc. and even though it's not open source it kinda has a similar community feeling. Every bit as feature-filled and capable as any of the industry standards.
I use Jellyfin for movies and TV shows, but never tried for music because I already had Navidrome set up. It is so good, really one of my all-time favourite pieces of software. It greatly repays a well-tagged collection, relying on embedded metadata only. Not sure how Jellyfin works here, maybe there is some ability to scrape album info from online sources (?), but I believe it's pretty strict about directory structure (one folder per album), which Navidrome doesn't care about.