or this "LINK FOR DIGITAL TACH"
There are also the hall sensor based tachometers that you can permanently attach.https://www.amazon.com/s?k=hall+tachometer
Found this. Might be helpful. Second image has the technical drawing.https://www.partsfortreadmill.com/icon-motor-pmdc-compatible-n1cpm-138t.html
yes
what kind of controller are you running ?? that might change the entire scenario with rpm , torque , heat , etc etc .
yes it will be a bit of work , might as well test it now , get it all set up on the bench before installing the motor.. So if for some reason , its not gonna pan out you can be planning ahead