Migrating Mojave Photos Library to Catalina

I am trying to migrate my Photos Library.photoslibrary file from my 2012 MacBook Pro running Mojave, to my new MacBook Pro which has Catalina, 10.15.1. I am doing a clean install so not using Migration Assistant. I copied the Photos Library.photoslibrary file over to my new MacBook Pro and opened it with the Catalina Photos app. It ran some kind of conversion process, and all seemed to be well, as it opened and everything looked fine. A few hours later, at least half the photos in the library suddenly changed to green and black blocks. I ran the photos library repair tool (option-command keys), which ran the library through the repair process, but when it finished it still had the green and black blocks in place of many photos. So I next copied the photos library over again, but now when I try to open it in Catalina, the Photos app just crashes.

Not sure what to try next. Photos is running fine on my old machine in Mojave, so I don't think there's a problem with the Library.photoslibrary file. Any ideas?