- Encrypted USB stick:
Pros:
-- Easy to setup
-- Long lasting and robust
Cons:
-- Costly, especially if you want to make multiple backup
-- Will it still work in 20yrs?
FYI there are some storage drive which claim itself have very long durability. For example, Sandisk Memory Vault claim up to 100 years and M-Disc claim up to 1000 years.
Do you have any other solution in mind that we can discuss to update this thread? 
Are you aware of existence of software or service which create address with custom spending condition such as Liana[1] or Heritage[2]?
[1] https://wizardsardine.com/liana/
[2] https://btcherit.com/