29 The verb is the Qal perfect שָׁלַחְתִּי (shalakhti), but a past tense, or completed action translation does not fit the context at all. Gesenius lists this reference as an example of the use of the perfect to express actions and facts, whose accomplishment is to be represented not as actual but only as possible. He offers this for Exod 9:15: “I had almost put forth” ( §106.p). Also possible is “I should have stretched out my hand.” Others read the potential nuance instead, and render it as “I could have …” as in the present translation.
30 The verb כָּחַד (kakhad) means “to hide, efface,” and in the Niphal it has the idea of “be effaced, ruined, destroyed.” Here it will carry the nuance of the result of the preceding verbs: “I could have stretched out my hand … and struck you … and (as a result) you would have been destroyed.”

In other versions

For by now I could have put out my hand and struck you and your people with pestilence, and you would have been cut off from the earth.

For by now I could have stretched out my hand and struck you and your people with a plague that would have wiped you off the earth.

By now I could have stretched out my hand and struck you and your people with a plague, and you would have been obliterated from the earth.

By now I could have lifted my hand and struck you and your people with a plague to wipe you off the face of the earth.

For now I could have stretched out my hand, and I could have struck you and your people with the plague, and you would have perished from the earth.

For now I will stretch out my hand, that I may smite thee and thy people with pestilence; and thou shalt be cut off from the earth.

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