Genuine question, forgive me if it's stupid: what happens immediately after you publish the blocks? Maybe I've misunderstood a step, but I think at this point you're in possession of a) whatever real-world goods or other currencies you bought with your bitcoins from the old chain, plus b) the same number of bitcoins on the new chain. Am I right that the value of bitcoin is now likely to crash rather quickly, as people inevitably realise what you've done? Is it just a question of completing the second spend quickly enough, before this happens?
The second spend in your double spend needs to be before the re-org is noticed. Make that spend a swap to Zcash / Monero and you can't be traced. So a full scenario would be:
* Swap coin for Zcash and start mining with 51%.
* Wait until your chain is longer than the main chain, and you actually hold the Zcash.
* Publish the longer chain, and immediately swap your spent coin for Zcash again. (At a different exchange just to be sure).
Now, you got twice the value of Zcash you needed, and due to Zcash shielded transactions can't be traced. You just have to hope that your shenanigans won't tank the value of Zcash.
People reacted in so irrational ways to any BTC news related to crypto (the "this is good for bitcoin" meme), that I am not sure if it would crash the price. On hte other hand a "bank run" on attacked exchange is likely. And the victim of attack will almost always be the exchange. BTW no matter what crypto fans claim ("code is law") getting anything from a service and then canceling transaction that paid for it by 51% attack will be a criminal act under many jurisdictions.
Considering that shorting crypto is probably better solution. Thought I am not sure if it is even possible to borrow required amount of coins.
