Both Carpathian and the West Slavic group is lackluster compared to any of the neighboring culture group so I would merge the 2 into 1 group. It would makes sense game wise as Poland, Hungary and Bohemia gets PU or are encouraged to conquer the other 2 in their mission tree. Would be good to make Bohemia and Hungary both formable tag just like Poland. In this case Nitra/Transylvania/hungarian cultured country can form Hungary while Glogow/Opole/Czech cultured country can form Bohemia.
This would honestly be the best solution.
West Slavic and Carpathian are both weak groups, making them into a unified Central European group would help a lot.
The problem with the OP is that his notion is that Slovak is culturally closer to West Slavic.
However it fails to realize that in history slovaks were just as close to both really.
Not only that, in the modern day the 4 countries, Czechia, Slovakia, Hungary and Poland are still very culturally similar and they were in history as well.
There are cultural differences, obviously, but the differences are better explained by the distance between Dutch and North or South German than to be a completely different thing.
Furthermore, the Kingdoms were at times unified under a single monarch and they mostly used similar military means, such as Hussars.
So I think it's clear that they should just form a single group. This would also help a long way with a defensive barrier against the Ottomans in the early game.