I need ETH, 'cause I plan to sell NFTs on Opensea, earning ETH.
For ethereum, you need to use another wallet.
The best is to buy a hardware wallet like Trezor. It's open source and can provide the maximum security.
There are also many multi-currencies wallets. But the problem is they are close source.
May I instead create my own seed-phrase, like
ThisIsMySecretPhraseThatNobodyButMeKnowsBecauseIMadeItUpAllByMyself
Technically, it's possible to force electrum to do so. But since that's not a standard seed phrase and you won't be able to generate a same wallet using other wallets, that's not recommended at all.
If you want to have a valid seed phrase with your selected words, you must select 11 words from
BIP39 wordlist and brute-force the 12th word.
Of course, it's not a recommended method and you should let the seed phrase to be generated by electrum. Humans are not good at generating random data.
Are there Any ETH wallets that would allow me to do that?
For ethereum I don't think there's any.
Again, it's not recommended to have a seed phrase with your selected words, even if it's possible.