iMac: Migration Assistant stuck via Thunderbolt

Trying to use Migration Assistant, between an older iMac and recent purchase. It gets hung at he point shown in the image. I have tried this several times, waited hours, and we get stuck here. Thunderbolt connection, and I followed the method outlined by Apple. Unning Catalina on both machines. 2014 27" model and 2019 27" refurb.

Thoughts...?