You can invest with bitcoin alone, I already made a cash out today and made profits. Just holding your BTC is investing because if you buy now and it will grow as you go on because the price trend of it is increasing, well you can apply it to altcoins if you want but always choose wisely.
Bitcoin is already good investment, In this case just buy bitcoin and it can be considered as our investment. Because bitcoin is the long, mid, short term investment for everyone.
As the OP said, if he had a 2mil USD, I think it's the great amount if we are starting
in cryptocurrency the world. But of course it will be make a great profit if we trade.
We can invest in trading platform, but don't put all of our funds in there.