Ok so they have now response to you after a week.
Is your funds came from doing trades or you just deposit that amount in a single transaction?
If you got it from trades then tell them about it. Your trading history will be your proof. On the other hand, if you deposited it on a single whoop, looks like you trigger the alarm. I have the same experienced in our local exchange and I just tell them where my funds came. But since no such thing as documented legal papers as proof where our BTC came (that is already erasing the purpose of cryptos), they just asked if I have a source of income or work and that's it.
Looks like your case will not be solved for a quiet period of time. If you are really eager just try to follow what they said. Your purpose is to get the funds so no choice but to deal on that even how mad or angry we are. After that never used Bittrex again and always read terms on other exchanges.