FWIW, I'd like to implement this but there's a limitation on Apple Music currently which requires to use the API for this for each individual track, while the old ratings are available locally, offline and in bulk. One key use in Longplay of those ratings/faves is for the various metrics, which requires accessing that state for every track of every album. That's typically thousands to tens of thousands of tracks. The number of requests required for that would likely get blocked or throttled by Apple. I've been kicking that can down the road so far hoping that it'll get exposed through the local mechanism.

An alternative, intermediary approach could be to expose some of that underlying functionality from Longplay through AppleScript, so that it's possible to write a script to sync that between Longplay and Apple Music. (I've checked Apple Music's Shortcuts support and that doesn't expose favourites.)

I second adding support for Favouriting , in addition or instead of star ratings. Whilst I prefer star ratings personally, Apple Music Cloud Libraries do NOT sync 3rd party ratings (ie made via the SDK api) back to the cloud. Marvis Pro is the same (and lets you set to show ratings as readonly if desired due to this apple bug). Ive also confirmed this myself with an app I wrote back when they hid star ratings completely. So I relunctantly switched to the favourites heart. Would be great to be able to flag a track in LongPlay with the heart too.

On a side note: even Apple doesn’t embrace album “favorite” functionality. Every couple days I discover that some of my albums have lost their “favorite” marking. It was reported to Apple many times by many people but they doesn’t seem to bother.

So I just created “Loved” collection in the Longplay app and added albums manually. And I feel much safer with that approach than if Longplay relied on Apple’s faulty functionality