I'm not a local but have kited in Croatia a few times so my 50c:
1. Viganj: between Viganj and Korcula there is a narrow channel which creates venturi effect and its almost guaranteed that there is at least Bf 4 each day, usually stronger in the afternoon. Bring a big kite/board just to be sure.
Busy spot, lot of surfers, no shallow water, sideshore winds, limited space to launch/land. Still one of the best spots in Croatia.
Korcula is a must see, there is a cheap ferry from Viganj, 1000+ yrs old beautiful city.
2. Neretva: I've never been there, but planning to as its supposed to be an epic spot. Thermal winds, gets super busy, shallow water as its part of the Neretva river.
3. Bol: Similar to Viganj but a lot more fancy, only one spot to launch/land and u need to pay for the local kite school (a few years ago it was the case). More boat traffic, less surfers, more expensive. Probably better with family than Viganj as lots of hotels are around.
4. Nin: I only recommend Nin if you not plan to travel further south as its mostly LW. So light sometimes I couldn't ride with a 15m2 foil and big board (85kg), but if its a cold front strong enough then Bora wind could kick in from the mountains and its epic. That is pretty unlikely to happen in july