Sorry, but a lot of the assumptions this plan relies on are based on anachronisms. And it's a transparent Polowank (though that isn't really the issue).
Well, let's assume the Przemyslids really snatch the Czech lands for themselves. OK. But it's kinda more dubious with Slovakia, for one simple reason : Our only historical borders that are really set in stone since the Middle Ages are the northern and western one. Where are you going to draw the line on what constitutes medieval Slovakia (aka Upper Hungary) back then ? The current southern border of SK was literally improvised during the 1920s, after an agreement between the former Entente states, the League of Nations and the "losing" states. A southern border could lead through anywhere - Horehronie, the Ipeľ catchment, the Danube...
My advice : Make Boleslaw Chrobry hold on to modern-day Slovakia, greatly increasing the territorial and economic power of the early Polish kingdom. Sure, it's a far earlier POD and the butterflies released would be gigantic, but I think it's a far more reasonable POD than the Przemyslids just marching into northern Hungary and taking it over night like in some game of Risk.
Alternatively - if the early 14th century POD is a must - let the Przemyslids make pals with all rebelling nobles in northern Hungary and sway them to their side and, eventually, put said nobles under their alliegance. A move that would need to be conducted in parallel would be constant attacks or assasination attempts on the fledgling Hungarian Anjevins. Also, shoo off any other competent candidates from the Hungarian throne that could retalliate militarily against the northern warlords and the Polish.
And what about : The eastern border. Where does it end ? Transcarpathian Ukraine ? Or even further ? Projecting the OTL easternmost borders of the Polish-Lithuanian Commonwealth would be kind of anachronistic, given how the OTL PLC only came into being in the 15th century.
Another, border-unrelated note : What will the newly acquired territories gain from Przemyslid rule ? I doubt the Kingdom of Hungary would take the loss of its northern parts lightly. They were considered an essential part of the realm since its founding. The north's cities and gold, silver and copper mines were becoming increasingly important in the early 14th century, even before the new economic boom of the 1330s-1340s. So, if the Poles would want to keep this valuable territory, they'd have to genuinely convince the local populace that being a part of Hungary is not worth it and they'd be better off under Polish rule. In essence, win over their hearts and minds, as well as their territory. Then slowly integrate their society into the existing Polish one and start reaping the benefits of the aforementioned mine deposits and growing wealth from craftsmen and (in this era, mostly German) rich citizens and patricians of the north's cities.