Belts don’t actually break in most FRC applications - stripping teeth from a belt is the failure mode that is seen most often. We stripped teeth off frequently last year using a 15mm belt on a 24 tooth pulley with a 6" wheels (6 wheel West Coast Drive). This year we used a 30 tooth pulley with a 15mm belt and 6" wheels and only stripped one belt on our P-Bot which sees about 100 hours of drive time. We also later realized that the drive axle was bent which probably did the most damage to the belt.
We have been using belts for a long time. I wouldn’t recommend using a 9mm belt unless your DT is really weak - but that kinda defeat the purpose doesn’t it
8 teeth per inch of wheel seems reasonable for a 9mm belt - but for a 6 inch wheel that is a 48 tooth pulley and that is huge and you have the potential of beaching yourself on them, or just wearing them out on the floor.