There are packages in GNU Guix for using Amazon S3 as a backup storage
host. For example, Restic and python2-s3cmd. It's not necessary to use
non-free software with AWS for this purpose.
Sorry, maybe my question was not asked properly. I was thinking about a cloud server where I could run a Linux distro, to have Guix package manager over it. My friend suggested AWS, but as I said, I don't have any problem in using another one if it is discouraged by the community.
And for storage, yes, I do use external hard drives now.