So this is unsettling. It finally finished one of my giant databases, after more than a hundred 503 timeouts. It got some good runs of 4000-7000 items at a time, but mostly batches of 200-500. So that was four databases down, one to go,
The fifth one was on a different Ma book (not M1, an Intel one) but still running Big Sur. Same iCloud account, same version of DTP3, same network, same connectivity. And out of the gate it synced for 15 minutes straight, at 16 connections, more than 32,000 items before it got a 503.
Once the databases get uploaded, the downloads to mobile devices and syncs seem to do fairly well with the CloudKit location.