Think about a Media Safe. Some are really expensive, but this one is not too bad. Just a really small storage area. https://www.amazon.com/First-Alert-2040F-Water-Media/dp/B000...

What about break ins? Someone enters your place and steal your NAS (and the Media Safe)...

This.

That's my primary use case for Amazon Drive. I have a robust rsync of the workstations and laptops to a NAS, and then to a second (incremental-only, no delete) NAS. Works great, but if the house burns down, or if someone breaks in and steals the computers, I want to ensure there's a copy somewhere.

If that is your main concern you could always put it on an external drive and put it a bank safe deposit box. I've thought about doing that for at least the very important things, perhaps even printing some important pictures too.

You just need another house to burn down.

Don't you have friends or relatives at a reasonable distance who can set up mutual backups on each other's home servers?

Yes, but nobody else with a FTTC internet connection with an unlimited bandwidth allowance (I'm in the UK). I have 2TB of data, so speed is important.

I don't have a single friend that has a home server. Most adults don't even own computers anymore, just phones and perhaps an iPad.

A good scenario is building a backup server/nas solution that you can put in a little cubby at your friends place. There's trust involved that you're not using their internet to hack the government, and you have to be mindful of their bandwidth/power costs. So not a rackmount server or even a tower, but something much smaller and very appliance looking. A nuc sitting atop a wd passport or their "my book".

If it provides them a benefit like an in-house plex server, even better.

Another option would be to rent a safety deposit box at a bank for $25 per year and store your backups there as flash drives. Cheap and very secure.

Of course it requires you going to the bank regularly to update the backup.