iCloud not downloading original photos to Mac
I have a photo library of +25,000 images in iCloud. I recently reformatted and reinstalled macOS 10.15 on my MacBook Air and enabled iCloud photo syncing using the "Download Originals to this Mac" option. However, after downloading a couple hundred photos, it has stalled. I have kept my Mac online and powered on for days with no change, although sometimes it will download a few and stop again. I have tried turning syncing on and off, restarting the computer, and various other obvious options.
I have macOS Catalina (10.15.1) installed on a 2013 MacBook Air with 300gb free on my hard drive. My photo library syncs fine with my iOS 13 devices.
Does anyone have any ideas on how to force iCloud to download all my photos to my Mac?
MacBook Air 13", macOS 10.15