I wouldn't trust USB flash drives with anything long term. Best archival method would be to print something out (perhaps an encrypted message in a QR code), have it put away somewhere secure, and use that for a key to unlocking everything else.
If we are talking just about credentials, you can just print the password and access instructions to a password manager and give it to the people you trust. This is one place were having a cloud password manager might be helpful, otherwise you would need to also provide the access to a device containing the offline manager (or a updated copy of it)
You probably don't need to keep a whole flash drive for credentials. Unless you also want to keep some other files secure without being on other devices
Or of course you can use multiple key storage techniques and have a 2 out of 3 or more type setup. It all depends on how valuable the information is.
You can buy a piece of 100x50x3 mm 316 stainless steel (thats 4"x2"x1/8" in freedom units) for around $5.
Engraving it is a simple matter of using a $10 automatic center punch and write the password out in dot punched letters.
If necessary, cut plate in N pieces with a hack saw, distribute among N people.
I always mark bicycles this way, dot punch my last name underneath the bottom bracket shell.