Hetzner is a great service, but what you're pitching is in no way a solution tailored for the aforementioned usecase.

We're talking long term cold storage backup medium, as in meant to last many years.

SX65, Storagebox, and Backblaze are not cold storage. SX65 would be $7000 over 5 years for 80TB without redundancy. You could get an LTO-7 or even 8 drive and many times SX65's storage for less, and have literal hundreds if not thousands left over for compute or whatever else with no recurring cost. Hell you could get an autoloader all-in-one tape library with tapes to fill it for less than that. There are absolutely scenarios where SX65/Storagebox/Backblaze/Cloud-hosted-storage makes sense and is a decent value, but this isn't one of them. If you want off-site cold storage, Glacier handily beats them with money and TB's to spare.

If you want always-on and available "warmer" storage, great, but that's an entirely separate discussion/usecase.

Yes I guess for 80TB and 5 years, LTO starts looking better. For 20TB, StorageBox is still ahead.

Per https://aws.amazon.com/s3/glacier/pricing/ it looks like Glacier costs $3.6 per TB per month, which is a lot more than StorageBox even not counting egress fees. Is there a cheaper class of Glacier that I missed?

Another idea is simply to buy a 29TB hard drive or pair of them to keep spinning, doing occasional integrity checks. I've had terrible luck using HDD's for cold storage but by now have had a few spinning in servers for 5 years without failures. Those are hosted servers in data centers though. Environmental conditions at home might not be as good. Two HDD's idling might use $100 of electricity over 5 years.

An LTO7 drive on ebay is $1000+, while new is still close to $4000. I'm dubious of used tape drives but maybe it is an ok idea. Hmm. New LTO7 tapes are around $50 (6TB capacity) so just barely ahead of HDD.

I wonder why there is no "LTO as a service" where someone has an LTO drive in a data center, and for a fee will write a tape for you and ship it to you.