Hi, I think that the Proton ecosystem can become better if all the mobile apps have the same basic functionalities because some options are available only on certain of them.

Proton VPN is the only app that allows for app' icon customization. Would be awesome to be able to customize the icons of all the other Proton apps between at least light and dark mode.

Proton Mail and Proton Calendar are the only ones that have the option to change the app language.

Proton Calendar is the only app that has a built-in feedback button, for sending a feedback directly in the app and not by redirecting users to the feedback page (like Proton Pass) or by having to search it online.
The dev team probably wanna receive feedbacks here if possible and in that case I would suggest that every app should have a button for sending feedback that redirect on this website.

Proton VPN doesn't have theme settings (it is always in dark mode). I can understand that the app colors and design are better in dark mode but light mode should always be an option for those users that normally use it and/or that have light mode set on the other Proton apps.

One good example that I found of a feature that all the apps have is the "Report a problem" (or "Report a bug") button where we can write the problem directly on the apps and send it to the team.
(I haven't tested Proton Wallet and my opinion is based on the other apps' functionalities and settings available on Android devices)