You're already thinking along the right lines by being cautious about how you handle your nsec—great! I would highly recommend checking out Amber. Unlike full-featured Nostr clients, Amber is a lightweight signer app designed specifically for secure key handling. It separates your nsec from your main client and communicates over NIP-46, which adds an extra layer of safety. You can pair Amber with other Nostr clients via remote signing without ever exposing your private key to them. That’s arguably the most secure path on mobile.