Are you just coding curious and want to play around with seed derivations or do you actually have plans to use such a system that you create to store your coins? I think it's important to realize that you are creating a problem where one doesn't exist. If you want a BIP-39 compatible seed, then generate it with a wallet that creates such seeds. You can still use that seed in Electrum. If you want an Electrum-native seed, generate one with Electrum. 

Electrum is trusted and open-source, and it's been around for years. You can even do everything in a completely offline environment, so your seed never touches a computer with internet connection.