Photos sync with iCloud
My Air M1 will not sync Photos to iCloud on battery power. It breezes through syncing while on power. Are there any settings that are being missed?
Have you enabled a very short sleep time? Or perhaps power nap? Any other power saving settings?
Try turning them all off to see if that helps
The only thing then I can suggest to try is to restart your mac in safe mode and try to see if it works then.
If it does, then start normally and try again.
Safe mode will clear temporary files and caches, and sometimes fixes odd glitches such as this.
See:
Start up your Mac in safe mode – Apple Support (UK)