Do the link speeds shown by Windows the same as you are measuring?
I have never needed to mess with the default settings for the router to get normal speeds. The 5.0-1 radio should be about twice the speed of the 5.0-2 radio.
It would be best, for us, if you would have the AX enabled so we could eliminate that possibility. Otherwise, test it with a strong signal and move a laptop around to see if the signal strength varies. For the Desktops, you might try changing the orientation or antenna positions to see if anything changes.
I have an AX 210 using the AXE11000 and in the next room from the router and getting a 1441/735 Mbps on the 6e radio. I also have the AX11000 which never had your situation but I am not currently using it.
Based on what you wrote above it sounds like you are testing with a single device. Are you able to replicate these results with any other devices?