Both the security phase and the private key of a wallet are very important. You can choose any one for your wallet. Because none of them can be hacked by a hacker. However, since there are 12 or 24 words in the security phase, you can memorize the phase of your important or main wallet, but in the case of the private key, it is never possible to memorize it. And even if someone sees it, no matter how talented he is, he will never be able to memorize it if he cannot copy it. That is why I give more priority to the private key. And I think the private key is more powerful security than the security phase.
But also one that is easier to forget or lose, private keys (WIF) are more useful for importing to another wallet or for sweeping, however some wallets provide the option to export WIF private keys, but be aware that you are dealing with something extremely sensitive and you shouldn't be encouraged to handle these private keys in an online environment that may contain malware.
When doing this, always make sure to handle private keys on an air-gapped, offline-only computer.
In your place, I'd invest in a hardware wallet (like trezor, bitbox02, coldcard, passport) so that you don't have to do any of this, you (like any wallet) should make backup wallet (writing down the mnemonic phrase) in a safe, offline place and you can optionally
extend the mnemonic phrase to a custom phrase (BIP39 - passphrase).