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.
You should never use a seed generated through anything other than the HW wallet itself if you're primarily using it on a hardware wallet. That is a non-issue as you probably wouldn't want to expose your seed (and passphrase) to an Electrum instance while using Trezor.
That wasn't a recommendation, merely an observation. Generally I agree with you, however there are ways to safely and securely create a Bip39 seed phrase on an offline machine that can be used with Electrum or a hardware wallet. Many here have warned about using Ian Coleman's Bip39 tool, siting the concern that a browser doesn't provide enough entropy. However, the Bip39 tool does provide the option of entering your own entropy, and /dev/urandom can be used create a HEX string with the desired entropy.
Please correct me if I'm wrong, but I don't see that as any less secure than allowing a hardware wallet to generate a seed.