Interesting proposal! A few questions:

How would you propose assigning your 8 bit field version field? Something like this?

00000000 - P2PKH at m/44'/0'/0'
00000001 - P2SH-P2WPKH at m/49'/0'/0'
00000010 - P2WPKH at m/84'/0'/0'
00000011 - P2TR at m/86'/0'/0'

What if I want to use a script type/derivation path combo which isn't assigned a version number? What happens then? And what if I want to use the same seed phrase to generate both a P2PKH wallet and a P2WPKH wallet?

Taking 128 bits of entropy generating a 15 word seed phrase using your new system, I assume you are feeding the full 15 words in to PBKDF2? Or are you stripping out the 128 bits of entropy and converting to a "legacy" 12 word seed phrase before generating your wallet?

I disagree with your suggestion above to indicate whether a passphrase has been used in the general purpose field. One of the main uses of a passphrase is to add plausible deniability, which is eliminated if you indicate in your seed phrase that you have used a passphrase.