Honestly i fail to see risk of quantum-computer towards BIP 39 mnemonic seed. There's no data which can be used by quantum-computer to perform attack. For comparison, Bitcoin address become vulnerable when it's public key is revealed.
Thanks. I agree with you. The best possible attack is probably simply the brute-forcing process which could be potentially (if QC will ever be that powerful enough) dangerous for 12word mnemonic seeds (
Grover's algorithm could brute-force a 128-bit symmetric cryptographic key in roughly 2^64 iterations). Using 24word seed is probably safe.