All the more reason you shouldn't be trying to make up your own seed generation system. You will almost certainly trip up somewhere and end up with something very insecure or very difficult to recover from.

There is already a complete program for securely generating Electrum seed phrases. It's called Electrum.

Then use it as a BIP39 seed phrase. If you want an Electrum seed phrase, then use Electrum to generate one. Trying to convert one to the other will likely just end in disaster.