There was message in Electrum that it uses different word list,
No, the default word list of electrum is same as BIP39.
There was message in Electrum that it uses different word list, because BIP 39 does not satisfy their security standards.
Don't worry about the security.
Whether your seed phrase is BIP39 or it has been generated by electrum, if your seed phrase is generated on an airgapped device, you are safe.
If you want to import your BIP39 seed phrase into electrum, just check 'BIP39" option.
I first considered to generate seed words by hand, but calculating checksum (12th word) seemed too complicated.
The checksum isn't the 12th word. The checksum is the last 4 bits.