not crowded and pebble/sand beach near trogir?

IME the "beaches" in Croatia (and in fact many popular places in Europe) are generally composed of pebbles, or rather coarse dark sand.

I believe many Americans in particular have an image in their heads that pops up when they think "fun at the beach." Those images are almost universally images of beaches with broad, fine white sand, lapped by crystal clear turquoise waters, and there's always plenty of free space to stretch out on the beach and claim as your own. Those "fun at the beach" images generally correspond to beaches in places like Hawaii, Mexico, Thailand, or the South Pacific. Beaches like that are not the norm in Europe (at least not in my experience, across many European countries). Now, you certainly can find sandy beaches with some nice characteristics in parts of Europe (I've been to a few that knocked me out). Haven't seen any like that in Croatia, though.

My recommendations would be to calibrate your expectations to match local reality. In August, I suspect you will find beaches in Croatia to generally be quite pebbly and a bit crowded. You might get a pointer to some that aren't entirely, but I wouldn't count on it.