Solo mine bitcoin with huge hash rates! That would be my advice. People have hit blocks solo-mining with far lesser hash rates than what you could afford with $3M. Also, the heat generated could be used for heating your home / office and you save on heating too (if you live in a cold place). Best of luck!
With the increase of block size, you need a very fast internet connection to do the solo mining. Otherwise, there will be lots of orphan blocks.
i doubt there will be so much different in the bandwidth, if the block size limit double then going by logic, you will need double the speed, not that huge, the client is not so speed connection hungry anyway, i'm not counting full node