50%, not 5%. 
You'll be pleased to know that ProtonDrive is underdevelopment, as also confirmed by their twitter account here: https://twitter.com/protonmail/status/988808782951133184.
Their CEO Andy Yen gave a podcast in April which said they were looking at release in one to two years: https://podcast.asknoahshow.com/124?t=2549
In the meantime you can always attach a document to an email and save it as a draft in a new folder you create called "storage" or something similar. I know that's far from being as user friendly a proper drive, but it gets the job done.
You'll be pleased to know that ProtonDrive is underdevelopment, as also confirmed by their twitter account here: https://twitter.com/protonmail/status/988808782951133184.
Their CEO Andy Yen gave a podcast in April which said they were looking at release in one to two years: https://podcast.asknoahshow.com/124?t=2549
In the meantime you can always attach a document to an email and save it as a draft in a new folder you create called "storage" or something similar. I know that's far from being as user friendly a proper drive, but it gets the job done.
I personally currently use Dropbox. Not a privacy-friendly alternative to Google Drive, but yea. Still looking for a better alternative that doesn't require me to sacrifice that much in terms of usability.
You could look in to https://cryptomator.org/. Disclaimer: I have not used this service. It's open source, and basically encrypts and decrypts all the files you upload to the cloud on-the-fly via their desktop software or mobile app.