You are overestimating West Indies way too much. You seem to forget the fact that this series is taking place in Pakistan which is why they always have the upper hand and the Windies batting lineup is pretty low quality.
This combo is the reason why Pakistan should comfortably win this series. 300 - 400? Nope. Even something like 250 should be enough against this Windies side.
West Indies bowling was OK. But their top and middle order batting is pathetic. All of their experienced batsmen are either retired or unavailable and they sent a very inexperienced squad to this series. Kraigg Brathwaite and Mikyle Louis are the best openers they could find, and that itself tells a lot about the lack of player depth faced by this team. I am not saying that there are no quality batsmen in West Indies. But 95% of them are T20 specialists and hardly anyone wants to focus on test cricket.