Looks good, Mael. I often wear a simple polo shirt with the kilt in the warmer months. I don't have a hat such as yours, but I see people wearing similar hats often at Highland Games down here in the summer - very effective for keeping the sun off your face, which can be a necessity.

Only thing I might suggest is that your embossed belt and buckle are a tad fancy for the otherwise simple/casual statement you are making. So a plainer belt might be a good choice - or I actually find it cooler to wear no belt at all. You don't need one, and without a belt it seems to me that everything feels less restricted, which is a bonus in high temps.

But that's just a minor suggestion, mostly because you asked for critiques. Really, you are doing quite well!