Oh, the potter and the clay!
Paul, a student of the OT was referring to:
Jeremiah 18
1 The word which came to Jeremiah from the LORD, saying,
2 Arise, and go down to the potter's house, and there I will cause thee to hear my words.
4 And the vessel that he made of clay was marred in the hand of the potter: so he made it again another vessel, as seemed good to the potter to make it.
5 Then the word of the LORD came to me, saying,
6 O house of Israel, cannot I do with you as this potter? saith the LORD. Behold, as the clay is in the potter's hand, so are ye in mine hand, O house of Israel.
7 At what instant I shall speak concerning a nation, and concerning a kingdom, to pluck up, and to pull down, and to destroy it;
8 If that nation, against whom I have pronounced, turn from their evil, I will repent of the evil that I thought to do unto them.
9 And at what instant I shall speak concerning a nation, and concerning a kingdom, to build and to plant it;
10 If it do evil in my sight, that it obey not my voice, then I will repent of the good, wherewith I said I would benefit them.
"And the vessel that he made of clay was marred in the hand of the potter: so he made it again another vessel, as seemed good to the potter to make it."
The clay was already marred, so He made it into another vessel to fit His purposes! (Pharaoh fits this criteria.)
Was Pharaoh and Egypt eternally doomed? Could he/it repent and be spared God's wrath?
Sure!
"At what instant I shall speak concerning a nation, and concerning a kingdom, to pluck up, and to pull down, and to destroy it; If that nation, against whom I have pronounced, turn from their evil, I will repent of the evil that I thought to do unto them."
You're arguing from a conditional statement. The second half of this verse is just as true under Reformed theology as it is under your theology.