Travel days

1a.
Nisan 15 → from Goshen to Succoth
Nisan 16 → to Etham
Nisan 17 → day of rest in Etham
Nisan 18 → to Pihahiroth (by the sea)
Nisan 19 → Pharaoh begins his march
Nisan 20 → Egyptians approach Hebrews
Nisan 21 → passage through the sea

That view assumes the trip the Red Sea took place during the week of Unleavened Bread.

That could be so, but most certainly not using the schedule shown above.
That schedule is based on a Red Sea crossing at the Gulf of Suez; but as we will see later it’s 100% sure at the Gulf of Aqba.
The distance between Succoth and that gulf is about 175 air miles. Needless to say it’s impossible they walked that far in just 1 day.

1b.

Exo 3:18 “The elders of Israel will accept your message. Then you and the elders must go to the king of Egypt and tell him, ‘The Lord, the God of the Hebrews, has met with us. So please let us take a three-day journey into the wilderness to offer sacrifices to the Lord, our God.’

3 day trip + 1 day sacrifice + 3 day trip back = 7 days.
For that reason it wouldn't have been suspicious they had 7 days food and water with them.

Does that prove the trip to the Red Sea would be only 7 days?
I'm not so sure because even with Pharaoh chasing them they still wouldn't have reached a place they could eat.
Maybe Moses wanted to pick up some food when passing trough the town of Jethro his father-in-law. We don't know.

Was that verse still relevant when they finally left after the 10 plagues?
By then the polite request made in Exo 3:18 has turned into a forceful deadly demand. They even took gold and silver from the Egyptians with them.

Num 33:3 And they departed from Rameses in the first month, on the fifteenth day of the first month; on the morrow after the passover the children of Israel went out with an high hand in the sight of all the Egyptians.

Exod 19:1 In the third month, when the children of Israel were gone forth out of the land of Egypt, the same day came they into the wilderness of Sinai.

It can be debated if ‘in the third’ month is 60 or 90 days counted from the start of the Exodus. But that’s not really important for the point I’m trying to make.

From the start to the crossing is about 200 air miles. In 7 days that’s an average of 28.6 miles/day.

From the crossing to mt. Sinai depend on where mt. Sinai is located but that’s also about 200 miles. But those 200 miles took them 53 or 83 days.
That’s a daily average of 3.8 or 2.4 miles/day.
Sure the terrain could be harder, they were tired, there was no hurry because Pharaoh wasn’t a thread anymore. But even then it’s very low speed.

2. 30 days

1. So the Hebrews went out of Egypt, while the Egyptians wept, and repented that they had treated them so hardly. - Now they took their journey by Letopolis, a place at that time deserted, but where Babylon was built afterwards, when Cambyses laid Egypt waste: but as they went away hastily, on the third day they came to a place called Beelzephon, on the Red Sea; and when they had no food out of the land, because it was a desert, they eat of loaves kneaded of flour, only warmed by a gentle heat; and this food they made use of for thirty days;
Josephus Antiquities of the Jews - Book II - Chapter 15

Exod 16:1 And they took their journey from Elim, and all the congregation of the children of Israel came unto the wilderness of Sin, which is between Elim and Sinai, on the fifteenth day of the second month after their departing out of the land of Egypt.

30 days can be about right, because then they started to complain about food and shortly thereafter they got quail and manna.

Elim was across the Red Sea for at least a few days.

3. ? Days

Exod 19:1 In the third month, when the children of Israel were gone forth out of the land of Egypt, the same day came they into the wilderness of Sinai.

It can be debated if ‘in the third’ month is 60 or 90 days counted from the start of the Exodus. But that’s not really important for the point I’m trying to make.

From the start to the crossing is about 200 air miles. In 7 days that’s an average of 28.6 miles/day.