the thing is.. you should not be able to get any biscuits if you don't get to the last room of the run. The option to return to the hub with your biscuits is a bug haha, so you should have the risk of choosing which rooms to go to with more or less difficulty to try and get more biscuits, there is also gambling rooms.
So yeah you could say the game should have been harder without this bug. And yeah you do get something when you reach to 999 biscuits. It's intentionally something no one will ever get to probably but if you would then you will get a silly little thing for it haha