There is no reason to translate Yam Suph as Sea of Reeds at all. This is the actual mistranslation. The word suph has two very different meanings and etymologies in Scripture. One is an Egyptian word, and the other Hebrew. Suph only clearly means reeds in three verses (Ex.2:3, 5; Is. 19: 6) In Jonah it refers to the end of deep where Jonah is going down prior to be being swallowed by a giant fish, and is so translated in the LXX. In 25 instances it refers to either the place and incident where God saved his people, ending the Egyptian pursuit of them and ending their sojourn and slavery in Egypt, or to the Gulf of Aqaba arm of the Red Sea. It is Yam Suph in all 25 cases except one, where it is simply Suph. Suph (reeds) is an Egyptian loan word, but Suph is also a completely different word (Hebrew and also in Aramaic) meaning and end. There is no reason to translate Yam Suph as "Sea of Reeds".
There is no reason to translate Yam Suph as Sea of Reeds at all. This is the actual mistranslation. The word suph has two very different meanings and etymologies in Scripture. One is an Egyptian word, and the other Hebrew. Suph only clearly means reeds in three verses (Ex.2:3, 5; Is. 19: 6) In Jonah it refers to the end of deep where Jonah is going down prior to be being swallowed by a giant fish, and is so translated in the LXX. In 25 instances it refers to either the place and incident where God saved his people, ending the Egyptian pursuit of them and ending their sojourn and slavery in Egypt, or to the Gulf of Aqaba arm of the Red Sea. It is Yam Suph in all 25 cases except one, where it is simply Suph. Suph (reeds) is an Egyptian loan word, but Suph is also a completely different word (Hebrew and also in Aramaic) meaning and end. There is no reason to translate Yam Suph as "Sea of Reeds".