That is one reason why it is bad.
Another one is that it leaks bits of the plain text.
Any of these 2 reasons is enough to deem that as a bad design.
So BIP-39 is a stupid and bad idea, saving the wallet's key in an easy and human readable format?
BIP39 is neither security by obscurity nor does it leak plaintext bits.
Still not getting it and missing the point. Your alternative is to write down 100-300 random characters on a piece of paper (have fun with that) and then require your family to be above-average computer literate to be able to decrypt it.
These are neither 100-300 chars, nor is it too difficult for an average person to decrypt it using a 3-5 step instruction.
It doesn't make any sense to create a shitty and insecure shift-cipher (which has to be taught how to use) instead for example just a BIP39 passphrase. Most proper wallets can handle this.
That would be way more easy to use than either AES or your
shift shit cipher.
In the end it comes down to two pieces which have to be stored: the secret and the (encrypted) data.
Whether this is your plaintext-leaking-mnemonic [data] with the instruction and dates [the secret] or simply the securely (non-leaking) BIP39 passphrase protected mnemonic [data] and the passphrase [the secret] doesn't matter. The difference is, one is secure while the other one isn't.