iCloud corrupting almost ALL of my library

I’ve had an issue with iCloud resurface now after a few years where I’ve tried to turn on iCloud syncing of my photos and it’s worked well when I’m turning it on for my iPad (has about 60% of all my photos) but the second I turn it on for my iPhone, photos and videos start being corrupted and different photos and or videos overwrite others. Apple support has gone through multiple rounds of collecting data from my phone but have never found any root cause for the issue as well as denying that there is an issue and just keeps blaming it on poor internet connection, which doesn’t make sense as my iPad can sync to iCloud with no issues at all. By now this has made me lose at least 5000 photos and videos, but as I’m paying for 2tb of iCloud storage and there is plenty of space left on my iCloud storage (>1,5 tb) whilst I’m running out of storage on my phone, I really want to get iCloud synced photos to work. I’m grateful to any help offered!