I don't think it will matter one way or the other. There are so many people there at any time, so even if people leave for vacation, there will be enough visitors that it won't matter. I have been there several times and they are pretty much the same. I was there in June/July and it rained non-stop (2 years ago when the Louvre flooded). I was there at the same time last year and the weather was midling (cloudy, some light rain, some sun). It could be hot, it could be mild.
I find going to museums first thin in the morning to be the best way to not be in a crowd. I loved the Van Gogh. Arrived right at opening at 9a, spent two hours, By the time we left, it was getting much more crowded but the two hours we were there felt like a special opening for a small group). I did the same with the Rijks. The FOAM photography museum is fun. The Dutch Resistance museum is well done. The Old Church in the Attic was awesome. I never travel along Damrack, and actually avoid central Amsterdam except to visit sites, because its so trafficked. I stay in the outer rings and spend all my time there.