BIP39 is neither security by obscurity nor does it leak plaintext bits.
It's an easy way to store your wallet's seed; it doesn't leak anything because it
is "the leak". I employ this same simple and easy way to store the seed words, not random 100-300 character Base64 encrypted gibberish. You still don't get it.
These are neither 100-300 chars
Yes they are, 128 in the example above by AES256 encrypting 12 mnemonic words with the password "blabla". Are you blind or intentionally obtuse?
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.
Most wallets generate your 12-24 seed words for you, without the possibility of using a passphrase, so no.