It is no more difficult to change out the chainring or the rear cassette on an e-bike than with a regular bicycle. My e-bike has a front 44T chainring and came with a 11-42t rear cassette. For me the power assist from the motor made the gearing too low and so I replaced the cassette with a 11-34t one that provided small gaps in the gearing. For a cargo bike I would replace the front chainring for a smaller one and this is a very low cost change to make in terms of the part and the labor.
Easy to calculate the inches of travel with one rotation of the front chain ring and the largest rear sprocket and then determine how much of a change will occur with a smaller chainring.