Cloud/destination agnostic apps that only use client-side encryption?

One of the requests I've gotten over and over again from a non-tech but still heavy computer user is wanting to store data securely: 1) at rest and 2) in the cloud.

There are services that offer "secure data storage" but what secure means to them doesn't really mean secure. Anyway, in this particular case, I'm curious if there are any apps that essentially do this already:

1) Use standard FTP/SFTP/S3 remote storage.

2) Allow you to generate/store keys in your OS keychain (e.g. macOS Keychain, in this case)—without storing them anywhere else

3) Upload files and have the encryption managed for you by the app.

Here the keys never leave your system and the data is stored at-rest, securely.

Is there anything like this that a non-techie could set up and run?