I just went through it on Catalina, and I don't feel any less in control than I did with the manual sync :)
Apple Music includes iTunes Match so when you turn it on on the Mac (or wherever you keep your master library), Apple will try to match all your music to their catalog and upload anything they don't know to their cloud (privately, and except missing music videos). I did that first to see how it went then turned on Sync Library on the iPhone. Apple Music will then sync your library via the cloud.
I encountered some issues when turning it on - there were a number of songs/albums duplicated (but small % of my library), which needed cleanup which took some time to work through but now it seems to be stable. That said today I lost all playlists on my phone for no apparent reason but was able to force Apple Music to resync by turning Sync Library off and on. I am sure other issues will pop up but for the most part I'm pretty happy with it for now, and personally I like getting rid of the manual sync.
Should be able to go back though I am sure there will be some sort of issue and there will be threads about that - any music downloaded from Apple Music is stored in its own directory in your library and presumably gets erased when turning it off.
This post has some more information on some of the issues I encountered https://discussions.apple.com/thread/250904926
This is the Apple support post on this topic https://support.apple.com/en-us/HT204926