Here is what to do.
Yes 1 (one) small flat spot works great. Forget about weakening the shaft to a point of a failure! (not likely)
2 (two) set screws only put pressure on the spots that engage the set screws.
I fly F5B with a 17x18 prop spinning max RPM's 16 cells 4200's on a controller that stops the prop on a Dime. Pulling 200-225 Amps for short 3-5 sec. bursts. No slippage ,AND you can get the prop off when you want to.
Cut a flat spot on the shaft half as wide as the shaft . Loctite the set screw ONLY with blue loctite.
With airplanes we taper the flat spot going deeper toward the motor .This way if the prop gets loose it has to lossen all the way up the taper .Chances are you catch it before the whole prop /spinners comes off.
If your hesitant about grinding on your motor the go to Home Depot and get a Fat nail and practic on it before you touch your motor..
Dyno Don