There are few alternatives to Proton Drive. Filen.io is the closest one in features. But it's a small company, so it development takes time.
Another alternative is Tresorit. Feature wise it is far beyond Proton Drive and Filen, with more advanced sharing possibilities. But it's quite expensive, closed source and uses Azure under the hood on the server side.
Filen and Tresorit are the only ones with Linux apps. Proton Drive can be accessed via rclone, but that is quite slow tbh.