Is there an alternative iOS to macOS music and photo sync tool? With macOS Catalina iOS sync has moved to Finder and became very unreliable, very slow, and even adding a single song starts a very long loop of syncing:

https://www.reddit.com/r/MacOS/comments/df2o4g/ios_sync_is_completely_broken_in_catalina/

Also when you have 20-40 GB photos and videos, you are looking for something better than iCloud Photo library or Photo stream. Like a better UX of deciding what is uploaded to icloud, what is on the mac (where space is possibly limited) and what is on the iPhone, and where it's clear, that "delete" button deletes from which of the three in the end of the day. 

But slowness and not showing a progress status is also a huge issue with both Music/Finder sync and Photos app.

Is there a 3rd party project out there, who got these sync tools right between iOS, macOS and icloud? I'd be happy to support them.
It is hard to answer your question, as it has multiple elements to it. You're both asking about syncing between an iOS device and a Mac (over USB or WiFi), and you're asking about syncing between a Mac and iCloud Photos. In addition, the conditions for the answer is really subjective - even though I might think that somebody got sync tools  "just right", you might think that they're not "right" for you.

For syncing between the iOS device and the Mac, I can recommend this third party project:

http://www.libimobiledevice.org

It works perfectly well for syncing. I think it's "just right" for my purposes, where the built-in syncing in macOS isn't enough.

For syncing between the Mac and iCloud Photos, there doesn't exist any 3rd party program that allows you to decide exactly what is uploaded to the Photo Library, and what stays on the Mac (outside of the ordinary controls provided to you by Apple Photos).