I found that Keil & Delitzsch Old Testament Commentary covers it nicely here: https://www.studylight.org/commentaries/eng/kdo/2-kings-19.html
They confirm that this is "The" angel of the Lord by providing Exodus 12:23, 12:12, 12:13. They also gave Isaiah 37:36. I also read 2 Chronicles 32:21, "And the Lord sent "an" angel who destroyed every mighty warrior, commander and officer in the camp of the king of Assyria. So he returned in shame to his own land. And when he had entered the temple of his god, some of his own children killed him there with the sword."
I went and found out why "an" angel of the Lord was used and the following is what Keil & Delitzsch stated;
"2 Chronicles 32:21
The deliverance: cf. 2 Kings 19:35.; Isaiah 37:36. The number of Assyrians smitten by the angel of the Lord is not stated, as it was not of importance, the main fact being that the whole Assyrian host was annihilated, so that Sennacherib had to return with disgrace into his own land. This is what is signified by the rhetorical phrase: The angel of Jahve destroyed all the valiant warriors, and the leaders and princes of the king of Assyria, and he returned with shame of face (cf. Ezra 9:7; Psalms 44:16) to his land, where his sons slew him in the temple. In regard to the facts, see on 2 Kings 19:37 and Isaiah 37:38. The Keth. מיציאו is an orthographical error for מיציאי , a contraction of מן and יציאי from יציא , a passive formation with intransitive signification: some of those who went forth from his own bowels, i.e., some of his sons; cf. the similar formation miyliydeey, 1 Chronicles 20:4."
They still confirm it was "the" angel of the Lord that did the "smiting." I base this on what happened to Stephen at Acts 7. Stephen (before his death) is ripping the Jews for what they did to Jesus.
Stephen says at Acts 7:30, "And after forty years had passed "AN" angel appeared to him/Moses in the wilderness of Mount Sinai, in the flame of a burning throne bush."
At verse 38 Stephen says, "This is the one who was in the congregation in the wilderness together with "the" angel who was speaking to him on Mount Sinai, and who was with our fathers; and he received living oracles to pass on to you." This is definitely "the" angle of the Lord, (the pre-incarnate Jesus Christ) at Exodus 3 and at many other places in the Old Testament functioning as a mediator for the early Fathers and the nation of Israel.
There is a way of determining/grammatically if "a/an" angel of the Lord appeared or if "the" angel of the Lord appeared. The chief grammatical function of "an" (or "a") is to connote a thing NOT previously noted or recognized, while "the" connotes a thing previously noted or recognized.
For example, "the" angel of the Lord first appears as the angel of the Lord at Genesis 16:7. Notice what he says and does. He appears many times in the Old Testament. "An/a" angel of the Lord appears in the New Testament at Matthew 2:13, Matthew 28:2, Acts 5:19, Acts 8:26, Acts 12:7, 12:11 and at other places. As a side note, "the" angel of the Lord "NEVER" appears in the New Testament.