I have been unable to get Sidecar to work on my 5,1. I have an updated WIFI and Bluetooth card. I see sidecar and It even shows my iPad but then it just flickers the iPad’s screen and throws an error message.That’s what I did. I have 0.7.4 on the less powerful MP5,1 and it just worked.
Edit: Big Sur 11.6 now running on the single processor MP 5,1 now too. Just needed to do CMD+OPT+P+R trick as per my other MP5,1. Running with Samsung 870 EVO SSD 2TB in the #1 drive bay (with OWC SSD tray).
Now I just need to buy the WIFI upgrade card to get my WIFI working again. Happy. I see side car in the system settings too. Nice!