"The maximum withdrawal you can do at one time is 5BTC.
For wins larger than this we pay out the winnings in installments according to a schedule which we agree on together."
just agree on together for an istant withdrawal of the remaining 3.7
Joking of course: how can they state that bolded part?
I mean: I asked for a bigger withdrawal than 5btc, so it is clear I want my btc istantly, but you denied it so it is clear we disagree on that.
If I was you I wouldn't play for big amounts of btc there anymore: if you like their casino and they will honour your withdrawal request I would suggest to play there keeping you eventual winnings far lower than 5 btc and keep withdrawing (wishing you luck) more often for smaller amounts than your doing now.
If you like to play big just change casino