Yes, you are correct. Creating an empty library worked in the past. As soon as I enabled iCloud photos, it started downloading. Placeholders, then thumbnails, then camera originals. I was just taking some wild ass guess to see if I could kick start it under Catalina. I could not.
You also make my other point. Weeks or days for it to do what it does is INSANE. It is clearly bad engineering. Hire some engineers from Dropbox or EtreCheck.
And when you launch photos, what do you want first? The photos you took this morning or all 22,000 of your photos goling back ten years synced again every time you turn off and back on iCloud Photos when it is stuck doing nothing. Although you cannot be sure it is doing nothing, because Apple provide no ongoing verbose logging. Hey, I am old enough to remember modems and they gave verbose feedback so you could troubleshoot where the problem was happening in the log-in script.
Why does Apple insist on making software for dummies? Fine if they want to create a simple mode, with the option of a verbose mode. But they just remove every bit of information that can help an advanced user solve a prolblem.
BTW remember when Apple had defined Error Codes you could look up so you could maybe understand why the program threw the error in the first place? Well, according to AppleCare that database of error codes on Mac OS hasn’t been maintained for years. While Apple has no problem deprecating functions, error codes going back to the PowerPC are still in the database. Every programming team just willy-nilly adds new ones.