Yes, $1m is no joke, I also have my reservations about the people claiming they are making bets with this amount, they must have been so rich to be able to achieve that. But since we can't verify as we do not even see each other not to mention verifying, we can just give it the benefit of the doubt. But I read from the last post of the OP as I was reading the thread upwards that he had some proofs attached with the IDs, well, we can't still verify, it is the companies involved that can do that. These days on the internet, anything you see may not be what it actually is, that is the issue.
Besides, I wonder how people are so confident and daring to be betting $1m online with a company you've never seen in the physically or have an agreed term with them and sign some papers, or issue you some kind of official VIP cards to make it better provable. I don't think that I can do a thing like that, it is too risky. If it is a physical casino, it is better, but online these days, you might be creating much more trouble than you expected because if you bet with $1m and win an insane amount, who will pay, or be held if not paid the winning? Well, betting with a well-liquid casino is still a good one, but still, the fear will be there, to be honest. To avoid much fear, it would be better to distribute the money among casinos if I were the person.
Someone said, that these proofs were fake. Anyway i can`t believe that someone ready to make such bet in unknown casino on big enough odd, and the same time ask an advice on internet board from unknown people. Even if they are well-known on this board.
So much strange things in one place as for me. Of course the OP can return with proofs and we`ll applaud him.
On a deeper thought, I believe there are reasons why so many people say such outrageous things on popular platforms, it is either they want to lure people into believing they are rich and when those people contact them out of greed, one thing will lead to another, but the end will always be a scam. For some other people, they just want to create controversy, they may not want to scam anyone but just want people to feel they are rich, that's how ugly some people could be. Nevertheless, there are truly rich people who are looking for ways to risk their money higher, regardless of the category this guy falls into, one should be very careful.
As for the casinos/sportsbooks that will allow you to wager $1m at once, I think that will be scarce, and I do not think that any online ones will allow that. The best is to divide the money into parts and wager the same with many casinos or sportsbooks. However, this is better played with sportsbooks, especially when the games are scheduled so that you will not be tense or distracted for any reason. But this is quite possible with physical casinos which I even believe the person of his status fits in best where gambling and entertainment will meet at the fullest.