Linux

Music organization: Musicbrainz Picard, also available for windows if you want try it before migrating to linux: https://picard.musicbrainz.org/

For mobile sync I use Navidrome+Ultrasonic via Subsonic API. Navidrome is a music streaming server, but you can predownload songs, not just livestream them, so it's possible to use it fully offline. https://www.navidrome.org/

Strawberry is still active. But I think you need Beets or Picard, as others have mentioned.

There are a handful of other tools that can help with sorting as well. I think Quod Libet and Ex Falso are available on may distros, I used to use Ex Falso for quick manual tagging and pulling in data from other sources.

As for the syncing, I couldn't help you there. I have used Strawberry for this, for sure, but I tend to drag and drop to my DAP.

I was in your position a few years back. I missed MediaMonkey when shifting to Linux.

I found Tauon media player was a pretty solid replacement for playing local and network files, but ultimately settled on running Navidrome server and Feishin as a desktop client. I haven't looked back.

For organising your collection, I'd look at using either Musicbrainz Picard (GUI based) or Beets (CLI, and it's a little complicated at first). I generally use Beets with Musicbrainz database, and the Discog plugin for anything not found by MB.

I haven't found anything that is a complete package like MediaMonkey, but with a bit of effort and once the parts are set up, it's so much better.

Is there any particular reason you don't like using Spotify?