The problem you are experiencing is related to Pump 2 diverter valve. The scoring is from dirt, sand, or cinders including shingle cinders from your roof. They are picked up by bare feet approaching the spa. The dirt is going into the tub on the bottom or your feet and settles in the foot well. Pump 2 is NOT filtered and is picking up what settles on the floor of the spa and is sent through the diverter system.
It appears you are aware of HOW to clean the diverter, now if you can only control what is being introduced into your spa. I would suggest a rug or towel placed on the top of the entry steps, the last thing the feet touch. The problem is on the bottom of the feet, especially wet feet.