Paki here. I can't speak about the ecologica; effects, but I suspect most analyses out there are underestimating the damage.
First stage might well involve TNW exchange, this is the defensive doctrine Pakistan has built with tactical BMs.
Beyond that...
At a minimum, tens of millions dead immediately. Followed by tens more due to the destruction of state infrastructure and radiation, fallout would spread elsewhere, not sure how far or where. Destruction of wheat and other crops represent a major portion of the world's food source, supply chains disrupted too, expect hundreds of millions of deaths in the subcontinent and outside. Giant amount of refugees too.
>Pakistan ceases to exist.
>India would exist but in a pathetic capacity, survivers might wish they had died.
Indian and Pakistani nukes are far more advanced now than the last tests in 1998, where both tested devices with yields between 15 and 40 kt.
Sufficient time has passed for them to vastly improve Uranium and Plutonium production (fissile material). And sufficient time has passed for us to build thermonuclear warheads, as all other nuclear powers managed in shorter time decades ago. Pakistan for a while was using tritium for a half-measure between the older nukes and thermonuclear capability, that's probably present even on the Pakistani side too now.
Both have ample delivery systems for nukes. Indian SAM systems, both current and future, that includes Akash, S-400, PAD and AAD, all will not be sufficient in stopping any meaningful number of incoming Pakistani warheads. Pakistan has no ABM capacity.
Basically, Indian and Pakistani warheads aren't limited by yield anymore, there as big as they need to be before losing efficiciency in yield increases.
Also a word on why most estimates show that we in Pakistan have more warheads than India does. It seems counter-intuitive at first, but it's basically because we have more targets in India, and they have fewer to target here.
