You might be interested in Peergos [0][1] (creator here) which has official Linux apps, is E2EE, fully open source (including the server), and self-hostable. It's also recommended by privacy guides: https://www.privacyguides.org/en/cloud/#peergos

[0] https://peergos.org

[1] https://github.com/peergos/peergos