Nsec ~> Cold Storage
Hoping one day there is an airgapped Nostr signing device.
If there is, lmk.
Discussion
I think there is. I seem to recall a hardware bitcoin signer that also holds nostr keys. If it doesn't actually facilitate air-gapped signing, it's at least 90% of the way there. You'd just need a companion app, maybe even built into Amber, that read the NFC, or the QR and executed the signing. I just can't remember which device had the Nostr key feature.
I found it. There's actually a few hardware wallets that support nostr keys. The Passport by Foundation, the COLDCARD MK-4 and the Q support nostr addresses, and LNBits makes a dedicated Nostr signing device.
There are two Nostr signing devices that I know of but neither of them have good client support, or work on mobile :(