Make sure your app has a progressive web app version that has feature parity with the store apps. That way, the app will work on phones like the librephone, and, if Apple or Google decide to kick you off the store, you and your users have some recourse. As a bonus, it’s compatible with open source — users can modify the app and install it without jailbreaks, root or (for now) sideloading.
React Native supports this (and can mostly be bundled with electron for mac/win/linux support).
Are there other stacks people can recommend?