https://www.gotquestions.org/God-harden-Pharaoh-heart.html

It could be that, as a result of Pharaoh’s hard-heartedness, God hardened Pharaoh’s heart even further, allowing for the last few plagues and bringing God’s full glory into view (Exodus 9:12; 10:20, 27). Pharaoh and Egypt had brought these judgments on themselves with 400 years of slavery and mass murder. Since the wages of sin is death (Romans 6:23), and Pharaoh and Egypt had horribly sinned against God, it would have been just if God had completely annihilated Egypt. Therefore, God’s hardening Pharaoh’s heart was not unjust, and His bringing additional plagues against Egypt was not unjust. The plagues, as terrible as they were, actually demonstrate God’s mercy in not completely destroying Egypt, which would have been a perfectly just penalty.

Romans 9:17-18 declares, “For the Scripture says to Pharaoh: ‘I raised you up for this very purpose, that I might display my power in you and that my name might be proclaimed in all the earth.’ Therefore God has mercy on whom He wants to have mercy, and He hardens whom He wants to harden.”

Exodus 4:21 is the setup. God says he will harden pharaoh's heart so that he will not let the people go.

It's mentioned numerous times that God hardened his heart throughout the narrative. He's not they only one. Deut 2:30, Josh 11:20, Isa 63:17. Ask mention God hardened individuals and whole people groups.

Paul confirmed this is Romans 9:17.

God raised people up, hardens their hearts fire they purpose of God showing his power over them in their destruction.

‭‭Romans‬ ‭9:16‬ ‭NKJV‬‬ [16] So then it is not of him who wills, nor of him who runs, but of God who shows mercy.

Exodus 7:3? Not sure if that is what he means lol personally I feel God already knew Pharaoh's heart, but God always wants for us to turn back to Him.

He trusted The Plan. Perhaps he was doing God's will, how can we know?