But I did not understand why the hell he collapsed on the court like a dead fish and showed everyone his tongue.

This is a stupid gesture, I do not approve of it.
Although the victory was certainly a big grief for the Djokovic.
It is goal celebration from FIFA game. There was lots of people who did not understand the gesture but there were also some who were thrilled by it.
Personally, I find i a bit strange, but I understand the idea and intentions behind it, so well done. Tennis is a sport that can use all the extra media attention and even few bad boys. It has been too long from last Kyrgios incident
Yeah, it was just a part of a celebration I guess. Kyrgios is gone for now, no more antics from him as compared to last year, not just in court but out court revelations (sleeping with a fan), and drama (Nadal bickering).

Anyone looking at the Istanbul Atp challenger tournament?
Just saw some odds, but I'm not familiar with the players name. Not sure if others are betting or skipping this one.
There's still a lot of tennis tournament going on around, just like the Istanbul ATP challenger.
But as you have said, this kind of tournament are relatively small so no known players are entering. Better just wait for other tournaments in the calendar like the Astana Open, and then we have Moselle Open or the Laver Cup in the US.