When you fit bearings you shouldn't hit the center of the bearing. I use a suitably sized socket which applies the force to the outer race. This will stop "brinelling" the bearing races causing notchiness.
Also, it should not matter how tight you do up the wheel nuts as this will not load the bearings as there is a spacer between them so only the inner race is clamped up. I think you have to accept that if you ride off road wheel bearings don't last that long even if they are good quality.
