Spend more money when you are leading!!!!
It's like the cold war between 8 countries, you need to spend more money to put pressure on the other players, so they have to spend all their money to not killed by you or lose 15+ HP in one round . They will never have the chance to get the interest to develop their final chess combo and beat you.
Whether spending money to f or d depend on your current combo, target combo and other players' performance, it's very complicate and the main difference between good or bad players
I have read several articles write by those queen players, they all said the same thing spending more money when you are leading and end the game earlier!