OK, what I call my master library is in the mini, and I add CDs once in a while. Then, once in a while I would like to add to my phone what is not still there. That is what I call Sync the mini to my phone.
I was doing this for years, ALL the music is mine, I have bought it through the years, using iTunes on a Mac Air tuning Mojave.
All I needed to do was to connect my phone and say Sync. iTunes would add to my phone the CDs that were not there.
The first time that I tried to do this with the new phone (Max Pro), iTunes added ALL the music to my phone, not just the one that was not there. So, in my phone, every album had every song twice.
Then some people suggested using Finder, same thing...
I started asking for a way to delete all the songs from the phone, that way when finder would add ALL the songs to my phone, I would end up with a full copy of my master on the phone. Somebody suggested delete The Whole App, I did but when I was downloading the app from the App Store, it would come with ALL the songs that were there before I deleted the App.
Then I thought that maybe buying Apple Music ($7.99/Mo) it could be done, because "without buying Apple Music the 'Sync Library' option would not appear".
I understand why they want me to turn it off, they don't want me to steal the music from Apple Music. I really don't care about the music in Apple Music.
Well, if I want to sync, I have to buy Apple Music but then when I try to sync, they ask me to turn it off.
All I want is to sync my music, I don't care about Apple Music or its content.