MacOS photos sync slow and unreliable

You described my experience precisely. Any solutions?

That is o.k. then. And Photos should definitely sync over Wi-Fi, as long as you are not using your iPhone or iPad as a personal hotspot.

Have you checked, if you have removed all files in a Legacy format from your Photos Library on your Mac? Photos may pause with the syncing and get stuck, if you are having older videos of photos in your library, imported on older system versions, that still supported the older formats. You may want to test all videos, if you can edit them or play them; videos with the codecs shown as H.264 or HEVC should be save.

To test a video, export if from Photos with "File > Export > Export unmodified original", then try to reimport it into a different Photos Library created for testing. Any item that cannot be reimported to a test library needs to be removed from your iCloud Photos Library. Convert these items to a supported format.

Older photos can also create problems. I had to remove all PDFs from my library. also some scanned TIFFs, where the scanner had some color sync profiles embedded, that are no longer supported.

The usual suggestion is to leave Photos running, but in the background. "In the background" means some other app, like Finder, in front as the active app. A laptop should be plugged in. (Well, so should a desktop, but that's kind of obvious, I think.) It's helpful to leave it plugged in, running, Photos in background, overnight.

Where is your Photos Library on your Mac stored? Is it in your Pictures folder or are you keeping it on an external drive?

Are you using Photos while you wait for syncing? Photos doesn't want to slow your work because it's talking to iCloud, so it is slower while you have Photos open and up front.

How long would you recommend to keep photos off while syncing?