Questions about visitng on Koningsdag

Hi, I had the idea of visiting the Netherlands on Koningsdag this year and I have a few questions:

1. I mainly want to visit because I love fleamarkets, and the whole country turning into one is just amazing to me. The question is: is it really as big as it's made out to be? Is it worth it visiting mainly for that?
2. Where would be the best place to visit. I'm from Germany, so bonus points if it's close to the border
3. How many tourists do you guys get on that day? Are hotel prices going to skyrocket?
4. Is public transport still working normally on that day? I heard the netherlands are amazing to travel by public transport.
5. Anything else I should be thinking of? Special events I could visit or such?