I think I have fixed the issue now. However, the price was high:
- Windows PC: all photo sync features disabled
- On my Windows PC I used the tool "CopyTrans Cloudly" to make a full backup of all my iCloud photos to my PC.
- Macbook: all photo sync features disabled
- iPad: all photo sync disabled, and when it asked me, if I would want to keep or delete all iCloud-synced fotos: delete
- iPhone: all photo sync disabled, and when it asked me, if I would want to keep or delete all iCloud-synced fotos: delete
- Then:
- On my PC using Cloudly to delete ALL of my photos and videos stored in iCloud (just two mouse clicks)
- There were a few dozen remaining photos on the iPad, I had to delete them manually.
- Some some dozen self-recorded video clips I kept on the iPad.
- The iphone was - after removal from iCloud - almost empty, about one dozen videos and less than 10 photos remaining.
- I waited a full day, iphone was still okay, no cpu problem with photo app.
- Then I re-enabled iCloud on both the iPad and iPhone.
- The few photos and videos synced both directions via iCloud and are now equal on each device.
- Since almost a week now my iPhone is fine and Photo App load in the battery stats is low, about 5%, and battery life is back to normal. Device temperature is constantly cool. :) :)
Up to now I was not bold enough to switch Macbook and PC sync on again.
My theory is, that there might have been possible only one or a bunch of photo files, which may have gone corrupt or whatever, which made the Photo app gone crazy trying endless loops of sync attempts. Possible a thing, which might not have been an issue for years, and since the iOS update in December, this has become a serious problem for Photo app and iCloud sync.
My plan for the next couple of months is, to only make Photo backups via USB cable and iTunes.
And also want to for you guys here in the forum, which have the same problem, for your feedback within the next couple of weeks, if you were able to fix your issues too, and to hear your experienes.
Good luck!