How do I consistently sync mp3s between Mac and iPhone without using Apple Music or iCloud?

Two Relevant Notes:

1. This is NOT about music downloaded from Apple Music or music stored on Apple Music. I use MP3s. I don't have any music bought from Apple. I store my music on an external drive. I sync only my Macbook Air and my iphone, not icloud, etc. I don't use genius or any other music service or music application from Apple. I'm sorry to be picky. I've asked this question before and gotten answers about Apple Music that seem not to apply to my situation and don't work. I don't know any other way to get relevant answers.

2.Sync DOES work if I don't use if for weeks, then try again. Burt then sync works one time. If I sync again a half hour later it doesn't work. This is not a connection issue. Wifi is working fine. I'm logged in on both devices. Nothing has change, but Sync doesn't work the second time when I sync two times in succession.

In responses to various questions like mine, I see mention of something called "sync library" that's supposed to be in music/prefs on Mac. Or maybe in Settings on iphone. It's not there on either platform. Screenshots are below.

Here's what happens. I sync some artists and playlists. Those sync. Then I want to sync some more I forgot the first time. Those don't sync. I closed and reopened Music on phone and mac. I forced a restart on iphone. I rebooted Mac. I turned iphone off and turned it on again. Now Sync reports it's "determining which tracks to sync," then syncs nothing.

What do I do to make my macbook air sync all selected music with my iphone every time I sync?

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[Re-Titled by Moderator]

Original Title: Mac Music and iphone Music won't reliably sync mp3s between ios 18.5 and mac sequioa 15.5 NOT Apple Music