What about NTRU? Apparently no quantum based attack has been found and the keys don't have to be as long as other quantum-safe crytosystems.

Two messages might produce the same 52 bit hash. There is no guarantee that whichever messages you put into the hash function will not produce the same 52 bit hash. Therefore there is no guarantee that you will find all possible message digests in 2^52 attempts. In fact, with a 52-bit hash function there is a 50% probability that two messages will hash to the same value if you try 2^26 different messages.

Or am I misunderstanding you here D&T?

Quantum computers are like space travels to nearby galaxies - it is fun talking about them and researching possibilities, but it will not happen tomorrow or next year. Worrying that Bitcoin or asymmetric cryptography will became useless because of Quantum computers is like being afraid of alien invasion.

Quantum computing makes me believe that in some alternate universe, I'm filthy fucking rich smoking blunts and surrounded by bitches all day every day. It also makes me believe that in another alternate universe, I've committed suicide by slitting my throat with a dull edgeless spoon.

Oh right, Bitcoin.
Bitcoin exists, is the global reserve currency, and hasn't been invented yet.
Bitcoin doesn't exist, Satoshi was declared dead upon childbirth, and the Bildeburgs are a homeless poor family of Ethiopians.
Pick one.

It will mean you still can't buy anything with bitcoin.

Depends if we see a slow integration where every system will be able to migrate and adapt to QC, then there wouldnt be such a problem to switch to QC mining. The real threat is if governments (or some rich corporations) are building it in secret and will have a huge advantage over majority of current systems.