Help! How do I clear my Apple Music and start fresh?

First, I don't have my iCloud syncing music from my Mac to my iPhone. I don't know if that's relevant or if I should do that or not.

Second, I moved all of my old .mp3's to an external hard drive and want to always play music from that from now on (when using my Mac).

Third, inside of my iTunes (Apple Music), there is a ton of artists listed that have exclamation points that I want to get rid of. Somewhere along the way, I must've lost those .mp3's. It's very annoying to have the missing files mixed with the files that I have located on my external drive.

Essentially, I want to restart fresh so that the only artists shown in my iTunes (Apple Music) are the ones that are actually on my external hard drive where my .mp3's are located.

Anyone know how to do this? Also, after this is finished, should I sync my iTunes (Apple Music) to my iPhone or would that not make sense since all the files are on my external drive and it wouldn't be able to play them anyway?

Thanks in advance!