Yeah, I've left it overnight several times. In the morning it's always frozen (something like "uploading 9543 items" and the number doesn't change for hours). I've turned off Energy Saving so it doesn't shut down. I've kept it as the front window. I guess if I could put it in isolation for a month and not use it, it might work. But this is my daily laptop. I use it and take it places.
I get the feeling it's treating the last 9000+ images as a single upload. If gets stuck, it has to start all over again and then gets stuck again. Maybe there's a bad file somewhere in the batch that is the sticking point. But how would I ever find that?
One work around I found was to go directly to iCloud.com and use the upload button to add images. That works. But I have to go through and choose the photos I want to upload as opposed to letting the software figure it out. Is that my best option?