Sorry to be negative.
I believe your idea whilst very attractive on paper, is likely to fall down on aerodynamics.
I've clocked up some decent miles with a flatbed trailer (Carryfreedom Y-frame)
https://www.wiggle.co.nz/carry-freedom-y-frame-trailer/
with a plastic 90 litre box strapped to it.
The issue I found wasn't so much weight up hills (you get off and push) but rather the slightest headwind.
A breeze that you'd barely notice with just panniers could turn your ride into something most unpleasant.
My least favourite touring memory was such a ride across the Hay Plains in Australia where a steady headwind put me back some days due to not being able to pedal into it even in my lowest gear (around 15 gear inches, so very low).
I can't imagine towing something more wind catching, like any of this threads included photos, even on very flat roads, into a breeze.