webdav access to proton drive
use a secure webdav link to access proton drive, e.g., to sync DevonThink from www.devontechnologies.com databases
Chris Knutson commented
NextCloud, Enpass, Joplin... it seems every bit of software that allows me to move away from trusting free services with my data support Webdav as one of the key technologies for easily synchronizing my data between devices.
There are a number of services I use that support WebDAV for remote storage and synchronization. Support for WebDAV in ProtonMail would allow me to store my app data securely and privately and avoid those non-private services. For example, EnPass is a password manager that doesn't store your data online, but allows you to store it in cloud services, including those that support EnPass.
And to save other important backups like password managers databases.