How can this PWM converter circuit be improved?
One major problem: IRS44273L is a low-side gate driver IC, but you are trying to use it to drive high-side FETs. Your chip doesn't have any boost circuit to provide a gate drive above VCC like you need for an efficient high-side drive.

However, as other comments point out, there doesn't seem to be any reason you shouldn't control your pumps with a low-side switch instead of high-side.