Hello LC-John.
We appreciate you sharing this detailed feedback, it is helpful for others considering this unit. The 22" wheeled trimmer is designed to perform well in lighter-duty conditions like dry grass and standard weeds, but as you have noted, components such as the spindle/mower ball (ground contact point) can experience accelerated wear in abrasive environments like rocky soil. Frequent contact with hard surfaces increases friction and can shorten the lifespan of that part. To help extend durability, we recommend minimizing direct ground pressure when possible, periodically inspecting the cap for wear, and replacing it before excessive degradation affects performance. Availability of high-wear replacement parts can vary, so it may be beneficial to check periodically or connect with Greenworks success for assistance in locating compatible replacements or confirming part numbers. They can be reached either using the online chat function, on Greenworks Tools page, or by calling 1-888-909-6757.