you can use wood ash, it won’t be a very wholesome or good soap but it gets the job done and is easy to make. Though of course if it’s easy to make then why would people buy much of it from you?

I think it wouldn’t be too hard to convince people to use it for cleaning off obvious messes, way harder to convince them that “oh you see actually diseases are caused by these tiny creatures you can’t see, but trust me they’re there and my product will get rid of them!”.