It's a hard choice, really.
History-wise, István and Mátyás are both in the top hungarian rulers/leaders ever. Also they are probably the two most well-known ones.
Although it would be great to have a later leader, I have to say Kossuth has to go, at least from this perspective.
On the other hand, art-wise Kossuth is one of the best leaderheads out there IMO. Ekmek did a wonderful job.
So while I would never want to miss that LH, I also can't imagine a hungarian civ without István and Mátyás...
Also, if we are thinking about removing the late leader, does an early leader instead really would be justified? In my mind slightly yes, but only by a tiny bit.
Both the pre 1000 era, and the post Ottoman era are significantly different from everything else in almost every aspect.
Did I say hard choice? It's an impossible choice

Obviously I'm more than biased, but in my head 4 leaders are the absolute minimum.
Early leader, István, Mátyás, late leader.
Then again, I perfectly understand your reasons and plans about civs/leaders, and the design that HR won't go beyond 190 leaders since you want to have unique trait-combos for every leader.