Can't be too careful about security.

I'm thinking paper wallets with the key locked in one or more safes might be a good way to go. Those Winklevoss twins might have the right idea: http://nordic.businessinsider.com/winklevoss-twins-cut-up-key-to-protect-their-bitcoin-fortune-2017-12/ .

Or spread your coins over different devices / exchanges / paper, so that if something fails, you don't lose it all.

I wonder how long it is before thieves start actively targeting user's devices and private keys. Just because it's on a piece of paper on your desk doesn't mean it's safe. Safe from hackers yes, but that's not the only threat.