I am currently using a highly complex method to store a set of 24 mnemonic words. Decoding the mnemonic requires 20 minutes.

I am considering why not use a simpler approach?
For example, writing down 15 words on papers, and storing the remaining 9 words on an encrypted USB drives and online emails. Certainly, both the paper documents and electronic file should be kept with multiple copies.

I ask chatGPT, it say that is secure. However, considering that AI models often give unreliable information, it would be better to seek advice from friends on this website. Thanks!

Adding passphrase is better, and some message is obtained here https://blog.trezor.io/is-your-passphrase-strong-enough-d687f44c63af