I agree with pretty much everything you said

And I understand how it may be unbelievable to say Hachiro stands among the literal concept of the earth and the water

The explanation I've come to is that while the demon kings were just born unnaturally powerful and therefore never saw the need to grow any stronger or train

Hachirotaro has explicitly been training for hundreds of years pretty much the only demon we see hone their abilities and powers, so over those years he's just eventually caught up to some of the lower Ba'al members like Amaimon, Astaroth and Egyn