Does that mean, after creating a new Electrum wallet and writing down the automatically
generated new seed, the computer could be rebooted without loosing any funds?
After the funds have arrived in Electrum, I will transfer them to the Relai app, which is
linked to my IBAN bank account. This makes trading very easy.
Of course, you wouldn't lose your funds because the seed phrase is your wallet and when you have your seed phrase written down, you funds is safe. After reformating your computer, your funds remains in your wallet and not in your PC. You can import your seed phrase with a different PC or phone entirely and your funds will be sitting inside your wallet.
Trading is risky, be careful and trade with only the amount of money that you can afford to lose.