Is it possible to use my own mp3s and Apple/iCloud Music Library simultaneously?
I've just discovered a Mac app, WALTR 2, that I've used to successfully (and very easily) copy some mp3s to my iPhone over Wi-Fi, and they now appear in my iTunes library as local files, with all the correct metadata and album art. iCloud Music still enabled the whole time.

(I am in no way affiliated with WALTR 2, just was having the same problem as you and wanted to recommend an app that works for me.)