I agree with the comments that a subscription model and delivery through the Apple's app store seem at odds with the 'with data longevity in mind' objective.

Requiring a subscription makes me ask what happens when you stop supporting the project? Relying on the App Store makes me wonder what happens when Apple decides to ban your developer account?

I'm not saying these are dealbreakers, but I'd like to have some sort of answer to these concerns.

Having a self-hosted way to download and pay for the app would increase my confidence level, even if I did decide to purchase through the app store. Similarly, there are 'subscription' apps that let the user keep using the latest version of the app if they cancel the subscription, they are just no longer able to get updates. You can also make a pledge to open-source the app if once you're no longer commercially supporting it.