And every transaction completely spends the source keypairs' funds.

So the only attack vector a quantum computer could use is:

1. Observing newly broadcast/unconfirmed transactions

2. Deriving the private key(s) from the public key(s)

3. Creating and broadcasting its own transaction using the stolen keypairs before the original transaction confirms (presumably with a higher fee to win the confirmation race).

Please correct me if I'm wrong.

EDIT: correction: every transaction completely spends any selected UTXO of an associated keypair, not all of the "source keypairs' funds". Thus the attack vector also includes being able to steal from any keypair that has ever made a transaction and also has UTXOs.