Hi Derbychap,
Thanks for reaching out using Apple Support Communities. Here to help.
It sounds like there may be additional Photos on the one of the devices that are not yet in the iCloud Photo Library.
Have you checked the status on the iPhone and iPad? On your iPhone, iPad, or iPod touch, open the Photos app, go to the Library tab, and tap All Photos, then scroll to the bottom of your screen.
Is it possible there are Photos on your Mac that have not uploaded to iCloud yet? You can compare the number of photos displaying on each device to determine the discrepancy.
Set up and use iCloud Photos
One possible workaround would be to upgrade your storage to allow the Photo upload to complete, then manage your iCloud storage and downgrade your plan if you can get the usage below 50GB again. The article below will assist with managing your iCloud storage plan.
Manage your iCloud storage
Hope this helps! Take care for now