Subscription models only make sense for an app/service that have recurring costs. In the case of Lemmy apps, the instances are the ones with recurring hosting costs, not the apps.
If an app doesn’t have recurring hosting costs, it only makes sense to have one up front payment and then maybe in app purchases to pay for new features going forward
Your suggestion is incompatible with the current infrastructure of either Google Play or Apples’ App Store. It’s nearly impossible to do that you’re asking without severely limiting the app.