This is interesting because I have been playing here for a few years now too and haven't had an issue although I have never attemtped to withdraw that amount of money. What I can say is there are a couple things you can do.

First go to askgamblers website and open a proper complaint against Bitstarz. They have a really good success rate with getting issues resolved similar to yours. AG will act as a middle man and you can present your case with the proof and Bitstarz will have a certain amount of time to respond.

If that doesn't work although it usually does, the casino is licensed so you can contact the commission and file a complaint with them as well.