Currently Proton makes all of its APKs available via protonapps.com; however it does not make the primary pages for the APKs easy to access or obvious. Additionally, Proton has been previously committed to following a more open-source modelling, publishing to Github. However consistency across Proton Apps is quite poor, and trying to access APKs via this method easily (in essence, as per an RSS feed) is not ideal.

I note that Proton partially supports alternative with VPN and Pass both on F-Droid. Whilst the GrapheneOS community often has issues with this, it is ultimately up to the user.

I would recommend that Proton however commits entirely to at least 1 complete solution to allow reliable and clear alternaive access to APKs outside fo the Google Play environment for its entire app suite.

1. Accrescent Store
2. F-Droid Store
3. Github
4. protonapps.com

Any of these would be fine, but it requires fully published and current APK files and the proper support to enable easy tracking and validation

My personal preference is Accrescent as a more rigorous privacy-centric "storefront", it would seem the missions of Proton overlap with projects like Graphene and Accrescent - and the benefit of the latter is that it would not lock people into Graphene either by default.