Good day Petercito,
The claim that God would be unjust would be false. And the reason is that, there is none righteous, no not one. All fall short of God's righteous standards. No one is entitled to salvation, it is a free gift for those whom God chose. For no one can come to the Son except the Father draws him. This would infer that God does not draw everyone. Therefore, those who don't come to Christ are still guilty of sin and the reward for sin is death.
So, God is not being unjust, for God has mercy upon whom He will have mercy and He hardens whom He hardens.
As Paul stated in His example demonstrating God's sovereign choice:
"One of you will say to me, “Then why does God still find fault? For who can resist His will?” But who are you, O man, to talk back to God? Shall what is formed say to Him who formed it, “Why have you made me like this?” Does not the potter have the right to make from the same lump of clay one vessel of honor and another for dishonor?
Regarding God's sovereign choice and Pharaoh He says:
"For this very purpose I have raised you up, so that I might show My power in you, and that My name should be declared in all the earth.” So then, He shows mercy to whom He wants, and He hardens whom He wants."