Still unsure why two Arduinos are needed unless the link above is relative to your project ?
Others have use the TX/RX pins with success to implement Bluetooth and pendants etc.
It could simply be you have two bad boards that were damaged somewhere along the process.
Yes GRBL is very serial intensive.
That is part and parcel of the whole GRBL scenario.
You can get "offline" shield additions to supplement that.
Those approaches tend to use an SD card or wi-fi etc.
Heads up Wifi does tend to be slower due to the amount of handshaking used.
Below my settings in LaserGrbl that have worked for all varieties of Arduino UNO
