I use Mega. They have the cheapest per TB S3 compatible service I can find, and they've actually been very reliable. I use Kopia to perform (and encrypt) the backups, so they never get anything other than the raw data and it's all done on a schedule.

I think I paid $300/year for 20TB, so it's not free, but it's offsite, well encrypted, and I'm pretty sure it will still be there if I need it next week.