Thanks - I appreciate the detail and hear you on the complexity and security risk it creates. I'm not yet thrilled about this route. I hope something better comes along.Let me preface this by saying, don’t do this. Don’t even think about doing it.
However.
You need to write down your external IP address, or better yet map it to a domain name using something like dyndns. Then you port-forward all of the relevant ports in your router to the time capsule. This effectively puts your NAS directly on the internet, without a firewall. Expect all of your information to be hacked (especially if it’s an older device) and anything you have stored to be exposed to the type of people who will exploit it. An especially evil person could even add files to your backup, so that when you restore, you restore with a virus. Like, really, don’t do this.
But then it’s just a matter of pointing your Time Machine restore to the IP address or URL instead of the NAS on your local network, and it will restore.
For now, maybe what I can do is iCloud backup and then have key folders that are accessed through a remote NAS. In this way, I can recreate the basics of my machine with iCloud's backup, which is built into the Apple ecosystem.