It is a very daunting problem no doubt, maybe the hardest one, but I also don't doubt that the end form of Starship will require way, way less than 20 000 man-hours per turnaround (as the Space Shuttle required) to refurbish the heat shield. They simply cannot afford that.
a. Cheaper, more durable material (stainless steel).
b. Cheaper, easier to manufacture engines.
c. Easier to use fuel (methane is much "tamer" than hydrogen).
d. Standardized heat shield with much smaller requirements for manual work.