Blockcore Notes allows import of private key, stored encrypted with a personal password (protected by Keychain), but the key itself is not in the keychain (username is the npub). There are also two ways to handle actions using the key, right now the user need to unlock the key for every single signing. I don't know if I should implement support for "autosign for 10 minutes" or something allowing users to not use password for each signing.

Nostr Connect and NIP-26 support will arrive soon too, then I think all bases are covered? Public Key, Private Key (hex and nsec, import with copy/paste or QR code scanning), Secret Recovery Phrase (mnemonic to private key), Extension (NIP-07). I'm looking forward to doing NIP-26 support.