nostr isn't an identity backup. nostr *is* the identity. here's how to create a backup of it!

run your own relay. that's it!

you can do this in a multitude of methods. here's just a few simple examples to get you started.

on Android you can run a relay on your phone: https://github.com/greenart7c3/Citrine 

on Windows you can run a relay on your computer: https://github.com/CodyTseng/nostr-relay-tray

on Umbrel/Start9/Mynode you can run a relay on your own server: https://apps.umbrel.com/app/nostr-relay 
https://github.com/Start9Labs/nostr-rs-relay-startos 

if you want a DIY method, you can run a variety of relays too: 
https://github.com/cameri/nostream (i use this one.)
https://github.com/hoytech/strfry 

good luck!