I also noticed that the original request is not complete release from bondage. Rather:
Then say to him, ‘The Lord, the God of the Hebrews, has sent me to say to you: Let my people go, so that they may worship me in the wilderness. Ex. 7:16
Go to Pharaoh and say to him, ‘This is what the Lord says: Let my people go, so that they may worship me. Ex. 8:1
And more specifically after the flies, from 8:25-28
25 Then Pharaoh summoned Moses and Aaron and said, “Go, sacrifice to your God here in the land.”
26 But Moses said, “That would not be right. The sacrifices we offer the Lord our God would be detestable to the Egyptians. And if we offer sacrifices that are detestable in their eyes, will they not stone us? 27 We must take a three-day journey into the wilderness to offer sacrifices to the Lord our God, as he commands us.”
28 Pharaoh said, “I will let you go to offer sacrifices to the Lord your God in the wilderness, but you must not go very far. Now pray for me
So, up through the hail plague, at least, the request was just a week or so to go make an offering to YHWH. All very interesting to me.
Last thought, at Ex. 7:7 we learn that Moses is three years younger than Aaron. First born children get a bad rap for the Hebrews it seems.