Paper. There’s a project called paperkey that allows you to store GPG keys on A4 paper. You could apply a similar approach to your age encrypted private keys or store them in plain text.
Modern smartphones have excellent OCR (optical character recognition) capabilities, so converting images of printed text back into digital form is now quite easy and reliable.
Personally, I use 1Password, and even they recommend printing out a PDF copy of your passwords and storing it in a secure location - like a physical vault. It’s a practical backup in case something happens and someone needs access to your credentials.