electrum seed is not bip39 so you can't use the coleman site to generate address specific private keys.
Thats true. At least for the mnemonic seed phrase.
But since electrum does follow the BIP44 specification, the xpriv can be entered into the
Account Extended Private Key field on
https://iancoleman.io/bip39/ to restore the private keys.
So a backup of the mnemonic seed phrase AND the xpriv would be the best choice regarding safety.