Your seed is designed to allow the user to access the coins with the seed only in the case of 2FA.
Yeah, if your seed ends up in google search results, 2FA will do nothing to protect your funds. A passphrase can be set up with along with a 2FA wallet, and just like above, that would help somewhat.
If your passphrase is long and random enough, there is very little chance someone would ever be able to be able to bruteforce it. I don't think Electrum limits the length of the passphrase. If it is long enough, then it would be equivalent to be bruteforcing without any prior information.
I don't think Electrum has a limit to the length of an extension, but some hardware wallets do. Trezor has a limit of about 50 characters, so if you want to add an extension to a Bip39 seed phrase and want it compatible with hardware wallets, that'll be a limiting factor.