I'm not sure what Music.app is not enough with modern iPhones. I sync my iPod Classic directly with Music.app on my Monterey M1 Mac Studio and it works great.
I have a single music library, not managed by any apps... just one named folder per artist, with one named subfolders per album. I do not stream music. I own all of my music outright. That way, my music isn't hostage to any streaming services whims. The music is also all DRM free (this took a little work to accomplish) and will play on any Mac.
I then just told Music.app on my Monterey M1 Mac Studio to import the top level folder and the rest happened pretty automatically. It absorbed the entire folder hierarchy and did the right things with it. I then did the same thing with my painstakingly developed playlists... I exported them from an older Mac and imported them in Music.app. Worked like a charm.
That said, while I can manage all of my music and playlists via Music.app, I cannot sync any of my older iPhones and iPods to it - just my iPod Classic. For my older devices, I sync them to the native iTunes on my Power Mac G5 Dual running Sorbet Leopard. That G5 had the same music folder set on it that I put on my Monterey Mac Studio and I imported it into iTunes the same way. From my G5 then, I manage my older iPods and iPhones.
By the way, I followed the same approach on my Sonoma M3 MBP and it worked well too.
So... I really think the simplest solution is Music.app for your current devices and a G4 or G5 running Leopard and iTunes for your older devices. A common music set is the key enabler.