Hey Frank, you should be able to have different sales page for each membership. The plugin doesn’t care which page a payment button for membership is placed on. As long as each payment button is hooked to the correct membership level it’s all good.
So I would create a sales page for each membership/product then put links to all of them on the “Join US/membership Payment page”. This way they can be accessed separately (by going directly on that sales page) or thru the “Join us” page that have links to all the sales page/products.