I use my Synology NAS pointed at Google Nearline (through the included S3 support!). GCS also supports customer supplied encryption keys, but I store the bytes as encrypted on the NAS box itself.

As others have said, if you're just trying to back up your Mac, take a look at Arq.

Disclosure: I work on Google Cloud (and get $30/month of credit, so my first 3 TB are free).