I dont see the speeds I will be doing needing much leaning .
I'm thinking more about the leaning you do to balance, especially at low speeds, rather than knee-down cornering.
You'll feel more of the weight with a high hitch as you're moving that weight about to balance while you ride - it'll make your bike feel heavy, hitched lower down you'll be aware you're pulling the weight along but your bike will feel like your bike.
For the axel position, starting off particularly, if the weight is seesawing about the axel of the trailer the weight will pitch backwards when you pull away and you'll loose traction, you want the axel far enough back that theres nose weight still as you pull away and also if you're climbing.
Looks like fun though - I ran a workshop building bike trailers and hand-carts for cargo so that the people of Lincolnshire could continue to trade if a tsunami wiped away all the civil infrastructure.