Auto‑mount encrypted memory card at boot
Encrypt the memory card with a key file instead of a password
Intro Disclaimer I: This worked for me. Chances are it will work for you, too, but this is not a copy-paste tutorial. Stay vigilant, messing up will be on you. Make a backup before you begin. Maybe you should also know how to perform recovery. Nota bene: "Turn the phone OFF" = press the power button...