Proton Drive Integration with Android Default Storage
I haven't yet found a way to set Proton Drive as a default storage location on Android within the "My Files" functionality which drives selectable save options for most file save actions. Obviously, Google Drive is available along with MS OneDrive, internal storage, SD storage and network storage. Having Proton Drive set here would make file management much easier on Android mobile.

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Krtek commented
as long as the integration is not on the expense of security
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Chloe Price commented
The lack of Storage Access Framework (SAF) integration in secure storage apps is unfortunately common.. >_<
Another competitor has the same problem for example: https://github.com/FilenCloudDienste/filen-mobile/issues/150
Proton could make a difference here