Miners aren't brute forcing keys to existing wallets and stealing the bitcoin, as that's effectively impossible even for the biggest mining rig (like, a mining rig the size of the sun couldn't do it in a trillion years)

Technically miners could start mining by trying to guess private keys, but there's no reason to because the expected value is so so much worse.

Can quantum techniques allow one to more effectively search for a specific private key to a BTC account?

Best practice for secure bitcoin accounts is to always send the entire balance when making a transaction, and have the "change" go back to a new address

I haven't published the public key to my bitcoin wallet. If someone sends to my address how do they publish the public key?

There are new output formats with taproot but they aren't mandatory and you can still send to addresses with no published public key

BTC addresses that have never sent BTC are not vulnerable to quantum computers as the public key is only sent when a transaction is made. The address you send to is a hash of the public key and irreversible even with quantum computers

It's it the number of operations which is the square root? (Presumably the different types of computer don't take the same amount of time per operation)

Mining involves guessing a salt which, when added to data for a single block’s with of transactions, makes the hash have a certain sum of zeros.

They are only the same in that they are using randomness to search for some number satisfying a given criteria. But, for example, you couldn’t use mining hardware to search for wallets with open balances. The mining hardware is specially optimized for one thing only.

or more briefly: never

We've shown time and time again that our undefeatable algorithms aren't.

Would you be comfortable if the Blockchain was frozen in time for a hundred years, after which you could withdrawal your balance? Would it be impervious over that time frame?

If the sum of the block reward and the transaction fees decreases, then that would result in fewer groups willing to perform proof of work calculations, and would be followed by a decrease in the proof of work difficulty in order to maintain ~1 block every ten minutes. That lowered difficulty then results in a lower cost to attack Bitcoin.

So, there must always, always be profit in running proof of work calculations. Not only that, but to maintain the security of the ledger, any increase in Bitcoin valuation must result in a proportional increase in proof of work expenditure across the entire network. It's an absolute disaster of a system.

The opposite is true of finding private keys.