If you use iOS, you can set music synching to automatically encode to mp3 when synching with the phone, that worked really well for me, for a while.....
Apple music, if you keep your own music, has completely gone off the rails for me this last year.
I couldn't sync for ages because it told me my device was full, it had just lost it's handle on the music, none of the syncing options would add or remove anything, eventually, the other night, I went into the settings for the music app and manually deleted everything and resynced and it finally started working again!
But then, 12 hours later after sync completes (Same as you, around 100GB of music), I discover the album artwork is completely messed up, almost no albums have the correct artwork, they've all been swapped around at random, all albums have the same artwork for all tracks, just that they're completely the wrong ones.
Checking the apple forums, it seems this is not uncommon, and has been happening for a year. I did actually notice that one or 2 albums were wrong but put it down to slight glitch I could live with, but having them all wrong is infuriating, I recognise many albums by their covers and it helps browsing to pick something out.
I really loved iTunes, especially for automatically storing the music in a nice structured directory format. I tried apple music briefly when it first came out, and the first thing it did was to screw up albums, splitting up compilations, getting the wrong editions of albums and started downgrading all my unencoded music, put me off for life, I'm quite happy managing it myself and don't have to fear losing favourite albums due to licensing issues.