You do not necessary need more fuel to slow down. Mars atmosphere can absorb few extra km/s above 5 km/s with typical landing maneuver as long as the heat shiel

You do not necessary need more fuel to slow down. Mars atmosphere can absorb few extra km/s above 5 km/s with typical landing maneuver as long as the heat shield can handle that. On Earth Starship must handle 8 km/s re-entry and potentially up to 11 km/s if direct landing from Lunar orbit will be feasible.