Why not land more guns from the fleet?

Would the British have been better off landing as many guns from the ships as possible?

Not by itself; unstable ground required engineered supports and battery positions, so extra guns without qualified engineers could become unusable or dangerous.

Heavy guns could not simply be dragged ashore and fired effectively from whatever ground was available. Batteries needed supports suited to local soil, and a mistaken foundation could bury a gun or fail violently under recoil. An improvised position therefore risked destroying the weapon or killing its crew rather than strengthening the siege. The fleet possessed artillery, but without engineers to assess and prepare the ground, landing more pieces would not automatically create useful firepower.