Mega uses end-to-end encryption, and its clients are source-available. The hosting is fully managed, and there is no self-hosting option.

https://mega.nz

https://github.com/meganz

Seafile is end-to-end encrypted and open core. They offer a managed hosting option, and you can also self-host.

https://www.seafile.com

https://github.com/haiwen/seafile

Nextcloud is working on end-to-end encryption, but the feature is not yet stable. It is fully open source.

https://nextcloud.com/endtoend

https://github.com/nextcloud