When the weakest link between the criminal and the cryptocurrency is a single person (the holder himself in this instance), that person alone would need to withstand all attacks and “rubber hose cryptanalysis”.
You could store passphrases in a hardware wallet in a bank vault in a small European country.
A little bit of irony here having to store your crypto related stuff at a bank to keep it safe.
Physical security for digital credentials is the main point here, that doesn't always imply a regular bank, many modern banks lack the bank vaults of yore in any case.
Tangentially, avoid showing up unannounced at grandparents house: https://www.youtube.com/watch?v=oZZmFG07OVs
I would have thought one of those libertarian seasteads or enclaves would be axiomatically the best place for such things?