If you were to stop the transfer, you couldn't guarantee that any of the data will be valid, so starting over in some form would be best.
WiFi is amazing.... but not for transferring vast amounts of data. Grab an ethernet cable for the next attempt. Oh wait - that's showing my age, and lack of interest in modern Macs I guess... Um - I guess Apple expects you to pay through the nose for an ethernet to USBC connector or something, do they??
At any rate, given the age of the previous machine, I'd suggest a manual transfer rather than Migration Assistant. I'm actually surprised it even said it would work. I thought it couldn't handle much more than 5-6 years apart.