Way better riding and way less edging.... and way better strapless airs but thats beside the point. Surfboard and bad knees.... I suggested strapless with smaller kites. You suggest powered boosting?
surfboard wise, yes bigger wider boards are gonna be more bouncy in choppy conditions, and a smaller narrower board is smoother through chop, but it doesn't really work that way in matching boards to conditions. Id posit that you can wreck your knees on anything depending more on how you ride it than anything else. Crank around powered on a needle and your knees won't last as long as feathering a smaller kite while surfing a bigger board. I fully plan on kiting into the sunset and once joints are in need of more consideration, its the boosting that will become less of a focus. Fortunately how you ride a surfboard in its element is an endless pursuit well worthy of your time. I plan on doing it with a quiver of surfboards and a kite or two.