Ad others have said, nextcloud won’t rescan or reindex on a reboot. no idea why sync thing does, and surely there must be some way to disable that, too. I’m still hesitant to recommend NC as it’s somewhat fragile, needs way more babying than I’m willing to keep up with and just does too many things, none of them anywhere close to “well”. File sync on real computers works solidly if you have a reliable connection (don’t get me started on Android).
Have you considered using a real media-hoster, like Jellyfin (or like a dozen others)? Jellyfin works fine for music (the are other music-only solutions though). There are plenty of clients that can stream, and have offline support (download a subset/albums/playlists) for things like laptops, phones, … The server can usually transcode audio formats that a client can’t play, in real-time, if needed.
Edit: I realize I wasn’t clear as to what this means in practice. You essentially get a self-hosted Spotify. Your library, run from your server, optionally you can connect to it from anywhere.
This sounds like it only boots Linux ISOs? I kinda need the ability to boot all kinds of images, only some of them Linux based.