The classic cargo bikes I know always had leaf springs. But it would take quite a heavy load to put those to use. There's also a bit of spring in the wheels and the frame, and of course in the tyres, with more weight of the bike itself.
I think that if you want to design a suspension that brings comfort at any load you'll have to do a very thorough job. I don't think you want a cargo bike and certainly not an electric cargo bike that feels comfortable and safe at high speeds but isn't.