Ask HN: How to create secure backups of my smartphone?

Both Android and iOS have in-built methods of creating backups that upload the data to Google/Apple. Both encrypt the data, but the keys are kept on their servers.

I would like to have end-to-end encrypted backups. For a while I just copied important files off my phone to my PC, but this is not an ideal solution, instead I'd like the phone to automatically create backups and upload them to wherever.

Are there any easy methods for doing this? I can't find any solid trustable solutions for this through Googling.