Photo (iPhoto) sync not working on old iMac (Sierra)

I have an old iMac running Sierra which uses iPhoto. I need to use the iMac CD/DVD drive to read and import some old photo discs to the photo library and then have it sync to my iCloud Photos. I’m able to import the photos to the library however the photo library on the iMac will not finish syncing to the cloud so that I can see all photos on all of my newer devices (iPhone, iPad, MacBook Air). It used to sync without issue, but now It seems to be stuck and won’t sync any photos dated after Feb 2024 including the photos that I just imported into it which are older than that. I’m now not sure how many photos on the iMac have never finished syncing to the cloud. I’ve tried logging out of iCloud and back in but that didn’t work. I’ve tried repairing the library file but that didn’t work either. What other things can I try to get the library on the iMac to finish syncing all of the photos? I don’t want to lose any photos and want to avoid creating multiple photo libraries so that I don’t end up with lots of duplicates.