Great question, and I'm looking forward to hearing the answers. Off the top of my head if you _really_ wanted to do it, then, probably both you and your partner getting an electric cargo bike and some kind of trailer [1] would be the way to go.

That said, if you're having to buy 300L of bagged sand _every week_ then no, it doesn't seem like a particularly fun time and frankly I'd stick with a car.

[1]. https://www.bikesatwork.com/bike-trailers