Just go to the Library tab in the phone app. That's where your own stuff is located.

I have over 8000 songs synced to my phone, 100% from local files on my computer.

I never activate the "library sync" BS that Apple tries to force on you, because historically it has replaced your copies with incorrect or "remastered" (AKA dynamically compressed to hell) versions from Apple servers.

I've even caught it switching "library sync" on without permission during an update.

Please re-read what I wrote. I know where the local stuff is. It doesn't fix the glitch that app is