Such a huge amount, do you want to buy all at once, I think you should not, as current market is already in the hype, and buying now will definitely going to make you lose in the upcoming bear market. I think you should do DCA while investing in BTC as it is the most proper way right now. Is DCA as 5 to 10 million dollars are more than enough to invest in the bear market?

It's just a thought, not a piece of financial advice. So DYOR before following it. Besides whoever will make a deal with you must pass Satoshi text. As according to my knowledge it's a hash-verifying procedure like verifying the public key etc. to confirm if the buyer or seller has enough funds. I think you should deal with some liquid site.