Do you know if gphotos-sync downloads the photos in their original quality/version?
The only thing that it gets wrong is that it cannot restore the file created date to video. But in the grand scheme of things, that's a nit.
I've been putting off a Google photos back up for years, and this was my excuse - waiting for this issue to be resolved. But I'm being a fool, 60% compression is better than losing everything.
I have just compared originals I had which were uploaded to Google Photos vs the ones downloaded.
Visually there is a barely perceptible difference in a side-by-side comparison on a high quality and calibrated monitor. I wouldn't have noticed if it weren't side-by-side and zoomed in.
On the file size, it's gone from a 6MB jpeg down to about a 1.5MB jpeg.
On the smaller files that difference is far less, but jpegs above 3MB seem to see the biggest reduction in file size.
Thankfully I have all of my originals that weren't taken on the phone camera so I don't mind this so much as running my exiftool script over the original archive produces the same naming convention and I can merge those folders and still have the originals I care about.
Sheesh though... such a thing for Google to screw up on.