Can't sync from Windows iTunes: "iCloud Music Library is On"

I have ~9,000 mp3s stored on my Windows 10 computer, visible in my iTunes library, and would like to sync them with my iPhone XS.

When I first did this it worked OK, but now iTunes just states "iCloud Music Library is On"

whereas before I had the option to choose which music files to sync (step 3 here).

I have made sure that iCloud Music Library is turned off in iTunes Preferences:

I now cannot sync any new music I add to my iTunes library. I've tried reinstalling iTunes, turning sync on my phone on and off and signing out and back in to my Apple account.

This happened around the same time I signed up for a free trial of Apple Music - I don't know if that's related.

A previous answer to this was to turn iCloud Music Library off on your iPhone, I don't have that option:

Any ideas how to re-enable music sync from iTunes to iPhone, not using iCloud?

iPhone iOS 15.5

Windows iTunes 12.12.4.1

Thanks in advance,

Will.