The Graphene OS project had a backup mechanism with Seedvault which is currently more or less deprecated. This is where Proton/ Proton Drive could step in.
As Graphene OS and Proton share some big core values, most prominently a focus on privacy and security, a partnership between the two projects might be beneficial for both ecosystems.

THIS feature would be VERY important to me as I currently do not have backup enabled either for Google or Samsung cloud due to security/privacy reasons.

It would be a great benefit to have my phone's backup data stored on Proton Driver with automatic backup. This includes contacts, text messages, apps, settings (wifi, Bluetooth, home screen, ringtones, etc etc), and so forth.

Currently I'm forced to use Google to backup my Pixel, which is less than ideal. As my phone is important to me, I do need a backup in case anything goes wrong. If Proton Drive could automatically take backups and then store them securely, that would be a huge improvement.

Our mobiles have become more and more important in our daily life and they contain a lot of personal infos and tools that make our life easier and safer.
However, break it, lose it or so and you are lost.
The only existing solution is to have a mobile back-up on Samsung cloud, Google cloud or apple, sacrifing our privacy.
It would be very interesting to have the possiblity to back up our mobile (contacts, messages, calls, applications...) on a secure cloud like proton and be able to reload it to a new phone when necessary.

Proton is already moving towards a comprehensive, secure, private service. That fractures in some places. Smartphone backup is one of those places. Google for Android and Apple for iOS vacuum up vast amounts of our private data whether we like it or not and act like a huge leak in the the secure barrier Proton provide. It is possible to de-Google now with many phones thanks to the 'e' Foundation among others and I would like to see Proton integrate with this trend.

This really would be a great addition.

This depends mostly on the device/OS manufacturers and not Proton. There's a reason Google only offers backups to their own service just like Apple with iCloud.

As someone else wrote but also for Android: "It would be great for backup emails, just to save them as a PDF directly to proton drive
And also be able to save attachment directly without downloading and the uploading to proton drive"

It would be fantastic to be able to easily backup/restore an Android Phone to a ProtonDrive instance. This could be easily done by adding a gateway between SeedVault app (available on LineageOS by default) and ProtonDrive (SeedVault already support such feature with a NextCloud instance).