I use kDrive from Infomaniak (their email and calendar/tasks aswell). It's not E2EE, but depending on the plan, you get access to webDAV and can use rclone/crypt and encrypt your data yourself if E2EE is important to you. If not, they offer a 1TB plan for 1,9€/month but without WebDAV capabilities. Besides, they are privacy friendly and their transparency and use of open source clients/tools makes me trust them very much. They are also very environment friendly. I have been using them for more than a year and couldn't be happier. Btw, I am not associated with them in any way.

I don't know, I have been using them for more than a year and am SUPER happy. I use their mail, their kDrive sync and calendar/tasks and have had nearly no problems. And when I have had a problem, I have gotten very fast email support, and they even pushed an update when I let them know there was a problem with the keychain acquisition on KDE. Honestly, it's the best experience I have had ever with any cloud provider. I even have WebDAV support!

Not really! You can find the source code for almost everything in their github (I say almost because I haven't checked if everything is in there, but I know the clients are because I've looked them up). Besides, aside from offering extremely competitive prices, they are privacy friendly (don't offer end-to-end, but you can read their privacy policy) and use a very ethical infraestructure. I seriously recommend you check infomaniak up; I have been using them for 2 years and couldn't be happier.

It is already open-source. All of the source code is on their github and, for docs, they use an implementation of onlyoffice very similar to the one in Nextcloud