If we're going to have a wedding anyway, I want it to be the best one with no regrets!
Literal
Anyway wedding [obj-を] do [nominalizer-のだった] [if-ら], regrets-won't-remain best wedding [to-にしたい]!
どうせ + のだったら 'if it's going to be X anyway' is a rhetorical conditional, accepting an unwanted or inevitable outcome as a premise before arguing for the best execution of it. 悔いの残らない is a relative clause 'one that will leave no regrets' (の for が again inside the relative clause). ~にしたい is the desiderative of the にする pattern ('want to make it X').