Photos on Mac not syncing with iCloud – ThermalPolicy: Must not proceed
The short answer is you want your Mac to get out of CPU use so that iCloud photos will resume. Thermally cooling the case and/or quitting all other processes that use CPU would be warranted.

Since you're technical minded - you can dump the thermal stats to see if you feel you have a hardware issue or can shape the workload to relieve a proper thermal pressure warning:

 pmset -g thermlog


The long answer is there are many throttles and other things that could go wrong with iCloud upload. Look at this reverse engineering of things in the plain files portion of iCloud file upload - let alone the more detailed Photos use that does face, moment, web app and more features in play like daily upload caps, uploads to shared storage being different than your iCloud storage and optimizing the versions when you run low on storage space.



Diagram from the amazing article by Howard Oakley - https://eclecticlight.co/2018/03/30/inside-icloud-drive-uploading-a-file/