Should the wheels get grooves after 6 years of usuage - or did we install incorrectly?
Not necessarily. The wear on the wheels depends on the amount of use, just like your car tires. The more you use it the more wear the wheels will have on them. Sometimes a slight wear pattern will look and feel much worse with oxidation buildup. If there a black build up on the wheels most of the time cleaning the wheels with paper towels helps. We have wheels that are 5 years old and after some cleaning are good as new.
Keep in mind these wheels are wear items that will not last indefinitely.