Did you ever figure out what you wanted for a bike? There are a lot of cargo solutions in Europe. And, a few scattered around the USA. I've been wanting to try building a cargo trike for some time, but use a homemade cargo bike + trailer. Eventually I may update the tow vehicle, or go to a pure trike.
I've experimented a bit with 20x4.25" FAT tires, as well as 16x3" (mini-FAT).
The Eugene bike co-op, (CAT, Center for Appropriate Transport, Human Powered Machines) made what they called a "tri-hauler" in the past.
They still have the shop, but I don't know how many cargo bikes they're making. They used to have an apprenticeship program which included building one.
The founder, Jan VanderTuin has been slowing down a lot over the last few years.
Most of their more recent trikes have been electric assist.
Their trikes are front wheel drive only, and they tend to build them around 20" BMX style wheels and tires. Schwalbe Marathon 20". For a custom build, one could potentially use any where one wishes.