20” hub drives have different windings and spoke counts from 26 or 700C wheels. Unfortunately, AFAIK, Bafang does not offer any 20” rear hub drives, only front ones, so not currently an option for your cargo bike. Not to mention they are not compatible with torque sensors (or would require a needlessly complicated, pricey, special controller and fugly display unit.) Perhaps though a front hub drive would work okay with your cargo/tandem bike. Others have had success that way; I have a 20” Bafang front hub drive on my LWB ecumbent that performs well but that bike also sports a TSDZ2 for good measure.