You don't need to build all of them and could still see which one people would buy. You could make 5 websites with some decent SEO, set up a little pitch/video

You don't need to build all of them and could still see which one people would buy. You could make 5 websites with some decent SEO, set up a little pitch/video for each application with a download button and see how many people click download on each. You just have an error message when they click download saying sorry, the product is currently unavailable. But you collect the data and see which one wins. Also you could have a comments/suggestion on each to let customers tell you if you're close but they really want something else.