Russia is truly setting up to be a hyperpower in modern times. The biggest threat at this point is internal instability, but their constitution is very much set up in a way to try to avoid that. The next threat would be a coalition of western powers attempting to contain it, but there's no configuration of that which seems feasible for the remainder of the 17th century. Hard to say what can happen after that. One thing that can be said though, is that so long as it can count on the Orthodox alliance protecting its southern border it would likely take the entirety of Scandinavia through Spain to reach parity in troop numbers. Without that unity western powers would end up needing to keep too many troops home to watch their borders to match the Russians.Is it weird that now I'm rooting for Russia?
Also, if they have 28 million people by 1640 (double their OTL 1700 population), then their 1900 population is going to be INSANE!