I think you're too focused on the risk of your backups being compromised. Do you store all your data only on encrypted hard drives with passphrases that you have to enter at boot time, on machines that auto-lock after a short timeout? If not, then you're probably at way more risk of data exposure through theft of "live" media and machines than backups, even stored offsite.
Not to mention the fact that a fire safe is a juicy target for Mr Burglar when you're away from home.
IMO the solution to the issue of cumbersome storage is to heavily encrypt, multiply duplicate and distribute your data.
I GPG-encrypt zip files, copy them to USB keys, and post the USB memory keys to my mother. Small USB keys are practically free now. She posts her backups to me too, along with outdated backup USB keys for re-use. This isn't all that frequent, so it serves for worst-case DR, but it has the handy effect of keeping old backups for quite a long time in case of accidental deletion not detected immediately etc.
I've also left an old laptop hard drive with photos with her, for example. Unencrypted because I don't much care if they get out, but it'd be easy enough to use an encrypted file system and store the keys elsewhere.
Key storage is the important part. Those keys must be in multiple places but stored fairly securely. If you lose the keys, your encrypted backups are worthless. A safe deposit box is an option, but it's kind of expensive and not necessarily as safe as you might like to think. I keep my GnuPG private in paper and electronic form in a couple of places, passphrase protected. Only I know the passphrase. So if I have a head injury when my house burns down I'm in trouble, but really, what else is there to do?
I also use SpiderOak for cloud sync of much of my data. Anything particularly interesting like key financial and identity records, passwords, etc, gets gpg-encrypted first. The rest is in the clear on my laptop, and I'm at much more risk of my laptop being stolen than of SpiderOak being compromised. SpiderOak offers versioning and deleted file tracking, which is very important, since many people's backup routines fail to properly account for files that are accidentally deleted/damaged at some unknown point older than the last backup rotation.
(I really should get a fire safe and run a spare SSD in there for up-to-date protection from more moderate incidents; that way I could real-time sync it from my laptop via the media PC.)