I would agree mostly. If you need to many iterations you might run out of money however.
In general I think refueling is a far smaller problem then the heat shield.
The heat shield is such a massive component that it's conceivable that the first design is flawed, and resolving the flaw requires a dry mass increase that cuts into the payload and makes a Mars return unachievable.
Refueling is a schedule risk, maybe the first design will fail to account for something and it will take a couple iterations to get it right. There is zero risk that the problem is unsolvable.