Why Did My Starred and Favourite Photos Disappear?
Favourites are recorded in the .json sidecars, not in the photos. Here is how to find them before you delete the sidecars.
Your favourites are in the export, in a field called favorited inside each .json sidecar. Photos you starred have it set to true.
Google Photos kept favourites as a flag in its database, the same way it kept dates and captions. It is not a standard concept that lives inside an image file, so nothing was written into your pictures. When you open the export in another app, every photo looks equal and years of curation appear to have evaporated.
Being honest about this: favourites are the hardest of these to carry across, because there is no universally agreed EXIF field for them. Some tools map them to a five-star rating, which most photo managers understand, but it is a convention rather than a rule. Before you delete anything, at least know the information is there.
The dates, times, locations and captions do have standard homes, and those are the ones worth securing first. The app writes them into every photo from the sidecars, so the bulk of what Google knew about your library travels with the files. Get that done, check the results, and only then decide what to do with the .json files. The first 500 photos are free.