Any recommendations for backing up (private) keys? Like pgp keys for example? I consider printing them on paper, as text or QR code. Anything better than that?
Periodically copy them to an encrypted external (thumb) drive and keep that in a safe place. Use two if you want to store one of them off-site. Piece of cake with LUKS and dm-crypt on any modern Linux distro. Just use a long passphrase that you can remember.