I followed XxBBVTMxX's good advice from the "helpful" comments... but it didn't fully work. I wiped the music clean from my phone successfully, and it initially deleted all 35GBs. However, it left 3 EMPTY playlists or playlist folders in my Music app on the iPhone 12 (NOT ones I created within the phone... positively came from my MacBook). I didn't like that it did this, but I couldn't find any option anywhere to manually delete these empty playlist folders from my phone (I assumed because they weren't created there, and ultimately I figured this would get cleaned up since my Music settings showed "0 kb" under "Downloaded Music". So I moved on with the process, despite have 3 empty playlist folders left on my phone, that seemingly couldn't be deleted.
Low and behold though, when I plugged in and re-loaded all 35GBs, half the album covers are still totally messed up. In what almost can't be coincidence, it's almost all albums from the 3 playlist folders (even though there were entirely empty after deleting my music) that had never left the phone. So, I really need to learn how to wipe EVERYTHING (playlists included) from the phone, and (hopefully) start over, even though it'll take another 5-ish hours.
What's worse though, is now that I've done this once, my iPhone will no longer let me delete the music again. It shows "0kb" under "Downloaded Music" again, but when I got into the Music app, all of the music FILES are still there (with the incorrect cover art)... just none of them are playable (all greyed out... if you click a track it tells you it can't play).
So,
1) How do I totally reset the Music app on my iPhone (delete ALL music, ALL playlists, start totally clean) again?
2) If I can manage to do this for a second time, is there a way to make 100% sure that there are NO playlists or playlist folders (even from the MacBook) remaining on the iPhone, since it seems clear that's what is causing this issue for me?
Thanks in advance for any help!