I do not use iCloud Music Library. I use iTunes just for sync'ing music to my device and my iTunes library consists of my own CD collection which I've ripped and added to the iTunes library over the years.
Various attempts to use iTunes to clean up storage -- for example by unchecking all music selection and "applying", trying to "sync" while connected to iTunes with no music selected and even connecting to iTunes on a new computer and sync'ing -- have managed to reclaim some of the storage that was being attributed to "Other" in iTunes. It has gone down from 22GB to about 10.5GB, but I can't get it down further than that.
Here's an example of what I see in iTunes. Plug in device and iTunes says 10.5GB used by "Other". Go to "music" screen (where nothing is checked) and uncheck the sync music to device box. It then shows in usage at the bottom that I'll gain almost 10GB. Then I click "apply", it does it's thing, but then the usage bar jumps back up to where it was. So it seems to think there's something to be cleaned up, but nothing gets cleaned up. Not sure how much sense that makes, but this "other" storage got really large when the library suddenly became empty, so I'm assuming a lot of that music is still on the device but the music app has lost track of it, and it can no longer be reclaimed.
I also tried a fresh device backup in iTunes to the local computer and then did a restore-backup. Afterwards, there was no difference in storage usage. BTW, when I look at the device storage on my iPhone (settings -> general -> iPhone Storage) I see this 10.5GB is listed at the bottom as belonging to "system" (as opposed to iTunes calling it "other").