Samsung Smart Switch

Mine did the same but ended up working, left it and took hours. Try the smart switch using wifi instead of cable, put both phones on chargers, close to your router and they will complete over night. I had 483GB worth of data 🙈
The best and fastest option is an ssd, i did that too using a 1tb ssd sandisk, copied the whole phone 483GB in like 15 minutes. But u can only restore back from that ssd.

I am trying direct through cable. Mobile to Mobile.

I had also experienced same when I was setting up S21Ultra last year. The smart switch app stopped 2 - 3 times then at 4th try, it worked.