I believe 250.54 is as close as a clear answer as you can get.
Using #6 not prohibited or required. Earth only as a return is prohibited.
Otherwise size the bonding conductor from the pump to the panel based on the size of the circuit that may energize the equipment.
Check sections 810.21 (J) and 820.100 (D) which are seperate structures that have ground rods. They require # 6
I would recommend # 6.
If the engineer complains about the cost of the # 6 explain that it is a pre-emptive legal fee. [Linked Image]
Alan--

Alan--
If it was easy, anyone could do it.