New iMac not synching Photos properly with iCloud

Our late-2021 iMac recently bit the dust so we bought a new M1 powered iMac. Now I'm having Photos problems.

The old machine had a small SSD internal, so the Photos library resided on an external drive. This was so I had room to keep originals local (as well as iCloud). It also meant the System Photos Library was not in the default place. This appears to complicate matters for Mac OS.

I migrated to the new machine via Time Machine, copying over my user information and directories. However, this did NOT copy over the fact that the system photos library was on an external drive. (Why would it not restore this information?). So I had to point Photos to the new library, enable it as the system photo library, and have it keep originals locally as well. And then it begins the very long process of making sure this photo library matched the one on iCloud.

That process has failed for me.

It is stuck with 58,628 photos and 1,845 videos locally. Several hundred were not able to upload. Worse, a few hundred photos taken after the old iMac died, but were taken on our phones, are showing up only as blank icons, as. placeholders. The Photo Information for them confirms these are the recent photos taken on our phones. But it will not download a preview. And if I double-click it refuses to download the original, giving an error that it cannot.

I've used Macs since 1998. I just want my Mac to "just work" again.