So as in title, Is it possible to buy really large BTC amounts? like a million $? What was the largest amount of BTC you bought at once?
Yes, that is possible but if you have also a lot of money. Everything is possible if you have a lot of money. I recommend that before you buy large amount of bitcoin just wait first for the right moment when the bitcoin price drop down it is the time to buy bitcoin. Don't ever buy with the large amount of bitcoin because you will loss some of your money if you do that.