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.
That is the most important think. You should create multiple backups ans store them in different places. Even if you loose one usb, you can get the desired private keys in another backup that you created. In addition to this, to not forget to tell about this to a close relative. This should be a person you can always trust. In case something unfortunate happens to you, you can expect that person to withdraw the money and distribute it between the family members you want. I know that young people might not like this idea but death is something unpredictable.