~snip~
I did two things:
I bought two cryptoSteel Capsule
1: XPRV (111 characters)
2: 12 words, (BIP32), Electrum.
just to be sure

The XPRV(111 characters) is the master private key, to be able to get the master private key you need to type this command below
wallet.keystore.xprv
After the wallet creation on Electrum, you can open the console and type that command and you will get the xprv or mater private key.
Actually, the seed phrase is enough as your backup, but it's your choice if you want both of them.