FreeFileSync Forum

That's about the same as what I described to be doing, using FFS.
And it's besides the point.

It's too easy to say it's 'crappy' Android. This has been the reality on 2 phones of 2 brands.
So 'maybe' concentrate on a possible flaw in FFS, which is still the question.

I only use FreeFileSync to synchronize MP3-files from my PC to my Android phone, one way.

That works. But replacing files, because of some changed properties, that has always been very arduous. It gets stuck on those files for a very long time, before performing the actions and moving on.

Best ...