POSSIBLE SOLUTION ?:
OK, I found a remedy to part of this issue, at least for my problems. I was unable to see playlists and songs loaded from my Library on my Mac after the 18 updates. I downgraded back to 17.7 just to have a workable option and that worked fine.
Basically, I backed up my entire phone to my Mac, and then did a factory reset on the device without transferring back my data back in. Once the refresh was done, I updated it to 18.1 and then restored from my backup. However, I still had the same issue where Music on the iPhone didn't recognize my songs and playlists, even though they showed up in Finder and on the Music app on desktop.
I then told Music to sync my library and stopped the sync before it could finish, and for some reason, it loaded my music and playlists from the desktop app as I needed it to. Not sure what part forced it to resync properly, but I now have a working app on the phone, including streaming.
tbh, I think I stumbled into this solution, so if you decide to try it, good luck.(Your mileage may vary)