Yes, i created my own Private, public key and seed phrase on my own with my own script.
But the option of just importing the seed phrase and Electrum makes it bech32 works for me.
I know Electrum is open-source but you would rely on the community to find something fishy in the code, you yourself won't go on each update and read the whole code of Electrum. That's why i wanted to create my own private key. Also, when using Electrum wallet to create your key, you are trusting their Entropy, which it can be VERY secure and everything. But i feel safer trusting my own entropy.
You seems confused about the private key and the seedphrase?
Seedphrase contains unique private keys(Many private keys that can only derived from your seed phrase) and you can't just import the private key into your Seedphrase.
The private key is just a single key and the seed phrase contains many keys.
If you want to add this private key you just generated outside of your current Electrum wallet I think you will need to extract the private keys from your seed phrase and then import them into a new wallet including the private keys that you generated outside Electrum.
But that's not the best way to protect your wallet if you want a secure way never import the private key into an online device I suggest import it into cold storage Electrum wallet and only import the public key to an online device with Electrum.