That is, by definition, not e2e encrypted.
If you mean this:
> So use a self-hosted open-source cloud backup system with a VPS?
then why not? It just needs to be set up to encrypt before upload, and decrypt after download, and have some means of sharing keys to other clients. Unless I'm being dumb and missing something?
If a 3rd party can be compelled to decrypt it, it’s not e2e encrypted.
Before they do that, it is e2ee. After they do that, it is not.