For the beach, this is what has worked at the pond at my parent's place. The sand beach there has been in place since 1988 with little or no maintenance other than what I mention below. The portion on land is basically flat and 4+ inches deep with sand. This slows/stops runoff and any sediment that does run off into the water is sand so it doesn't cover your submerged sand with dirt/silt. The portion in the water is a similar slope to what you have which stops it from washing down. About 1 time each year dad sprays with glyphosate and runs the garden tiller on the land portion to remove weeds and loosen the sand.
