hmm, ok - thanks for the replies so far. Makes sense. Nothing is especially safe in this game, I suppose! Although, I guess I am more worried about my Nano and its seeds than I used to be about cold storage keys, tbh.
Nothing is especially safe!? You had better be worried about being
killed by a flying fire hydrant than about bruteforce of Bitcoins cryptographic keys. It has happened at least once somewhere that a man was killed by a flying fire hydrant. Bruteforce of the type of crypto used in Bitcoin has never happened, and never will.
(More to the point, be worried about your computers security, theft, etc. Bruteforcing of keys does not happen.
Stealing of keys happens oft. But if you use hardware wallets and cold storage, you are already better protected than most people.)