I don't see how you could work with a growth team on the side but if you are searching for a website with existing traffic maybe you could do marketing for a popular open-source project you like. It's not the ideal solution you described but it would allow you to learn marketing, even if it's by yourself. And you won't even have to make a product.
True. Writing an eBook is a lot easier than software.
But what I really want to do is help out an already established website with existing traffic. Rather than build my own from scratch.