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/

You can use any media management solution on your PC and sync that to your devices using https://syncthing.net/

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.

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.

My music workflow is the following: I'm using dynamic playlists based on last played timestamp. If a song was played, it gets a new timestamp and is removed from the playlist. Now a new song comes automatically in to the playlist where the timestamp doesn't exist or is older as x-days. That's easy to setup on strawberry and other applications. This playlist will be synced via whatever you want to your phone. In my case a SFTP service to keep it wireless. On the phone I use the same playlist with every player you want. Additional I'm using lastfm to scrobble the played music. This keeps the last played timestamp on the phone and can be synced with strawberry. I don't know if other applicants can do the same.

Sounds complicated at first but after initial setup it's a automatic process.

Not a great replacement but Plex with Plexamp might be an option for music. I'm not sure it can handle audiobook or podcasts metadata automatically. It will host it anyway but you might have to find the metadata externally or import it manually.

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