You're overthinking it, you can be successfully storing your private keys by putting them onto a USB (or a few USBs) and them putting that USB into a small, fireproof safe, and maybe making a couple copies of that USB and storing them in other locations as well. Write it down on some paper if you think there's a real risk of someone trying to snake your information with a virus or whatever. There's no harm in doing something like that and having multiple backups is the best thing you can do since it's all gone if you can't get the information back.