So you need the help of an experienced strategist? I'm always here.
As Hungarians, you need to take the rebels from around you, in such a way that it will give you good staging areas for your campaign against Byzantine, as that is, after all, the only place a hungarian king can go.
If you go to poland, your borders become stretched and your men get bogged down in a frontier province from which you can't attack anyone really. If you attack HRE, you will also get your borders overstretched. Only place you can attack is Byzantium, as there you can advance very long (all the way through turkey to egypt and westernmost northern africa) without stretching your borders. After that you can get enough surplus to go to russian steppes, and from there flock the polish borders. Rest is obvious.

Always calculate the required forces for expansion. What counts is not the size of enemy army or their generals, what counts is how many borders each province has. For example, if you advance from Hungary, take greece and other regions there, and stop in constantinople, you have only one border province in that direction, only one province that needs your troops. If you don't take constantinople, you have two border provinces, both requiring castle upgrades and troops.

And about Byzantines, yes, they may look impressive, but throw a bunch of crazy spearmen and archers at them and your cavalry in their rear (after they're tied) and you win. Of course, there is a bridge battle against them when you launch the attack, but that's what jobbagy are for die my stupid peasantly men. Take 3 units of them along, put one to cross the bridge, and press stop when they're on the middle. Enemy archers will deplete their arrows, and if enemy infies close in you kill them with both spears and your own archers. Do it correctly, and you will be able to take constantinople almost for free after the two harder battles in greece and the other.
Oh, and if you're doing some rapid advance, always consider the option of butchering the prisoners. If you are advancing against poorly defended yet deep enemy teritory you don't want to kill the same people 3 times in a row