So it has a dedicated music app?

Finamp, but I’m pretty sure you can get a stream URL to use in any client too.

It has music filtering?

Define “filtering”. You can browse by artist, genre, etc. just like in Plex.

Good 4k/x265 performance?

Haven’t tried 4k myself, but I don’t see why it wouldn’t. I think x265 depends on the playing client, not the server.

Has a third party (or built in) utility that shows me streaming usage?

Yes, on the builtin dashboard.

Allows me to limit remote users to streaming from a single IP address at a time?

Looks like the feature request for streams is still open, though it does have an option to limit concurrent sessions.

https://features.jellyfin.org/posts/59/account-based-stream-limit

https://features.jellyfin.org/posts/1444/limit-the-number-of-simultaneous-streams-per-user

Let’s me watch something together with another remote user?

Yes, it’s called “syncplay”.

Has an app for most any device (like Plex or Emby) that does NOT require sideloading?

I know it has an Android app in the Play store. I haven’t looked on other platforms because that’s all I’ve needed.

Has built in native DVR steaming/recording support?

Looks like it. Never used it myself.