I've been doing some testing on my end and found some things:
1) Any Non-Apple Music music I add to my library, like music I've recorded with our band and added, gets uploaded normally and any changes I make to any tracks listed as "Uploaded" happen across devices almost immediately. Almost as soon as the Music app starts updating iCloud Music Library, I can see these tracks show up on my iDevices immediately, although they're not available to listen to or download on those devices until the Music app finishes uploading them. This is fast and how everything worked for me in Big Sur.
2) After adding any of my own purchased music (Ripped CDs or BandCamp) to my Library that gets Matched & shows across iDevices, and then change Album titles or any other metadata, DOESN'T sync immediately and sometimes can take hours to be reflected across devices.
3) Any Apple Music I add to my Library in macOS isn't being reflected on my iPad or iPhone right away. Same with adding tracks from my iPhone or iPad, they're not showing up in macOS for what seems like 10-15 minutes or longer, and sometimes I'm having to reopen the Music app to see any of those changes.
So, Uploaded Tracks seem to reflect any changes made immediately while Apple Music & Matched tracks do not.
I've also got a very large library (87,714 tracks/931GB), and I'm sure that's not helping things, but still. This process was better/faster on Big Sur.
And, to be fair, This "syncing" issue seems to happen among other apps that utilize iCloud or sync to other devices. Again, this is just what I've noticed, but I've found other apps like Mail, Contacts, Calendar, syncing Bookmarks & Reading List in Safari... All of ''em seem to have some degree of issues "syncing" across to every device. I'm leaning in to everyone's "Music Syncing" issues as being more macOS related as opposed to just the Music app. Hopefully things get better when the Music app is native. I'm just not ready to commit to running another beta on my main machine.