Is it possible to etch a QR into glass? That would be more fire resistant and wouldn't age at all.
Today I shill an old project of mine, unrelated to Nostr: https://github.com/dtonon/papiro
Papiro is a very simple tool for storing any data in physical form by printing qrcodes on one or more pages. I created it a couple of years ago for a personal need; maybe it can be useful to you too to improve your backup strategy. Try it :)
Why QR Codes on paper?Because paper seems the most resilient and cost-effective storage medium on earth, at least for low volume data, and QR Codes have an interesting error-checking system. So it is a nice backup solution to pair with digital ones.
Discussion
Glass is extremely fragile. If you need to go further steel is a better solution.
Someone already think about it for seeds storage: https://seedhammer.com