Hmm the macOS photos folders have this many files so I’m not sure it’s an edge case to sync this many files between at least 2 clients. And the issues you mention are real even when syncing relatively static large folders.
But you’re right that there are many such error cases to handle if you want a real iCloud / Dropbox alternative.
But you’re right that there are many such error cases to handle if you want a real iCloud / Dropbox alternative.