I would think the easiest way would be to use a complex enough passphrase along with any seed phrase backed up on metal, and then move all your coins to a new wallet if you need to dispose of the back up. Then it is unlikely anyone would be able to brute force your wallet even if they obtained your discarded back up, and even if they do, you have only lost your privacy and not any coins.

The next easiest option would be to "write over" the seed phrase, so to speak - stamp every letter of the alphabet on top of every character already stamped on each washer. They will quickly become unreadable and therefore unrecoverable.