iCloud Photos Library + Migration Assistant

But my new computer is now uploading all 26,000 photos again to iCloud -- this took weeks the first time it uploaded everything.

Is this normal or should I delete my local library and just re-download all pictures from iCloud?

It is the expected behavior. When you enable iCloud Photo Library on a new device for he first time, the library will be merged into the existing iCloud Photo Library by uploading all photos again and comparing them.

See: Why is iCloud uploading my entire iCloud Photo library again after I moved its location?

Downloading from iCloud to an empty library will be much more efficient. But if you do that, you will lose the projects. If you do not have any projects to save, that will be fine. If the library in iCloud has been uploaded using Photos 3.0 on High Sierra it will contain the named faces, so they will sync back from iCloud.