Agreed about getting a cannon, but you may've left something out on your design. That, and iron ammunition might be a bit too short ranged. Enter, the C&P design from the last thread!
Djer 6’’ cannon
Djer is a new cannon for the Nafuna military. It fires what are essentially massive cartridges, upsized to be 6’’ in diameter. They have a primer and propellant in the a metal casing, with projectile at the other end. They are loaded from the breech of the gun, with firing happening by pulling a rope that releases a hammer that strikes the primer. The barrel is rifled.
The breach and the barrel rest of rail, allowing the entire assembly to move backwards when fired. There are two static tubes on top and below the barrel. One is simple tube filled with oil, while the other is a double tube, filled with oil on one end and pressurized air on the other, with a mobile seal between them. When fired, the gun moves backwards on the rail, pulling two pistons inside the sealed tubes. This causes the air to be compressed even more, as the movement of the piston causes the oil to push against the seal, moving it against the air. The gun returns to the firing position as the air seeks to uncompress, assisted by springs. This helps to counter the recoil of the gun.
This movement also reopens the breach, ejecting the spent cartridge, allowing the weapon to be rapidly reloaded.
EDIT: Given the better state of our tech, going for a rifled breach-loader seems like a decent plan.