Update.
First: the long odd photo file names. After digging a bit into this topic on my Mac and PC, I would think, these long file names seems not to be the reason for the sync issues of the Photos app. I found may files with long hexadecimal names, also some with easy readablewords and even brackets - "(" and ")" - up to 51 characters long. However it seems to me that especially Whatsapp uses those hex names for photos since years. I found old files back from all the years, where I never have had any CPU load issues. Also the other photos with names given by some photo editing apps, are older than my current issue.
Therefore I think its not the filenames, that causes my problem.
Second: It seems, that I have made a promising step forward during the last two days. For the first time since two months, since yesterday my phone stayed cold and Photos cpu load under 50% and a few minutes ago at 10% and Whatsapp at the first place with 20% and another app at second with 13% !!!!!!!!!!! Wow!!!!!
So what did I have done? After I have backed up all my photos offline to my PC, and have had stopped all iCloud photo sync, I deleted all the photos in iCloud in on all my devices (iPhone, iPad, Mac, PC).
Then I wiped my iPhone and installed a fresh new setup ==without restore from iCloud== and waited a day before I logged into iCloud again. And after iCloud login, I enabled photo sync again and the photos came back to my phone. Shared Albums are still off. No Photo stream.
Then the CPU load was high again for longer than a day, I guess, thats normal full-sync activity. Since yesterday however it is falling down again, device temp goes is down, and battery drain is back to normal.
By now it is still too early for giving all-clear, however it is looking promising. After a few days I will let you know about the situation again.
Just a thought for now: as a last resort I would remove all the photos from ALL my devices and start at zero photos in my Apple world. It is really a poor sign from Apple, that they do not properly address these serious issues of their customers, thats not premium at all.