I will also like to add that a pricing model like concepts app may attract more users. They have a subscription plan. But they also provide one time purchase bundles of essentials and professional tools. This type of purchase is available on a;; the same kind of device. E.G if I buy these bundles on an ipad I would be able to use these on every ipad that I log into with the same email.
Or we can make the pricing model like an pdf reader app caller liquidtext. This app also has a life time purchase offer where you can use it fully and get free maintanence for life . But, it gives you the choice to buy a new extra upgrade if you want to. These upgrades or niche features they release periodically. Also we buy this upgrade once as well.