From where do Jehovah's Witnesses derive the doctrine that Jesus was and now is Michael the Archangel?

To add a bit more to what was covered above-

1- Is there anyone who would better fit the description of "Chief messenger" than Jesus himself? Well, that's what the "archangel" is. The term means "Chief messenger."

2- Compare the name of Michael "who is like God" with John 1:1, and the word was with God (ho theos) and the word was theos (godlike).

3- Genesis 3:15 “15 And I will put enmity between you and the woman and between your offspring and her offspring. He will crush your head, and you will strike him in the heel.”” (The seed of the woman will crush the serpent’s head, commonly understood that Jesus will be the one to defeat Satan)

Matthew 13:41 “The Son of man will send his angels…” Matthew 26:53 “Or do you think that I cannot appeal to my Father to supply me at this moment more than 12 legions of angels?” (Jesus is the commander of the army of angels)

Matthew 24: 30-31: 30 Then the sign of the Son of man will appear in heaven, and all the tribes of the earth will beat themselves in grief, and they will see the Son of man coming on the clouds of heaven with power and great glory. 31 And he will send out his angels with a great trumpet sound, and they will gather his chosen ones together from the four winds, from one extremity of the heavens to their other extremity. (The Son of man will command the angels with a great trumpet.)

Matthew 24:15 ““Therefore, when you catch sight of the disgusting thing that causes desolation, as spoken about by Daniel the prophet, standing in a holy place (let the reader use discernment)” Daniel 10:21 “However, I will tell you the things recorded in the writings of truth. There is no one strongly supporting me in these things but Miʹcha•el, your prince.” (Michael is a heavenly prince)

Daniel 12:1-2 “During that time Miʹcha•el will stand up, the great prince who is standing in behalf of your people. And there will occur a time of distress such as has not occurred since there came to be a nation until that time. And during that time your people will escape, everyone who is found written down in the book. 2 And many of those asleep in the dust of the earth will wake up, some to everlasting life and others to reproach and to everlasting contempt. (Michael is the great prince standing on behalf to bring escape (salvation) for those written in the book, and raise the dead into everlasting life or reproach)

Joshua 5:13-15: 13 When Joshua was near Jerʹi•cho, he looked up and saw a man standing in front of him with a drawn sword in his hand. Joshua walked up to him and asked: “Are you on our side or on the side of our adversaries?” 14 To this he said: “No, but I have come as prince of Jehovah’s army.” With that Joshua fell with his face to the ground and prostrated himself and said to him: “What does my lord have to say to his servant?” 15 The prince of Jehovah’s army replied to Joshua: “Remove your sandals from your feet, because the place where you are standing is holy.” At once Joshua did so. (Michael is the heavenly prince, leader of Jehovah’s armies)

Revelation 12:7-10 7 And war broke out in heaven: Miʹcha•el and his angels battled with the dragon, and the dragon and its angels battled 8 but they did not prevail, nor was a place found for them any longer in heaven. 9 So down the great dragon was hurled, the original serpent, the one called Devil and Satan, who is misleading the entire inhabited earth; he was hurled down to the earth, and his angels were hurled down with him. 10 I heard a loud voice in heaven say: “Now have come to pass the salvation and the power and the Kingdom of our God and the authority of his Christ, because the accuser of our brothers has been hurled down, who accuses them day and night before our God! 11 And they conquered him because of the blood of the Lamb and because of the word of their witnessing, and they did not love their souls even in the face of death. (Michael leads the angels of heaven to conquer the dragon/serpent/devil/satan (establishing one being can have many titles) with the blood of the Lamb, which leads to a declaration of Salvation. Michael does battle, but it’s The Lamb/Jesus’s blood that brings Salvation, which Jesus is supposed to bring.

John 5:25: “I tell you the truth, a time is coming and has now come when the dead will hear the voice of the Son of God and those who hear will live” (The dead rise at hearing the voice of the Son.)

1 Thessalonians 4:16: because the Lord himself will descend from heaven with a commanding call, with an archangel’s voice and with God’s trumpet, and those who are dead in union with Christ will rise first. (It’s not just that Jesus is described as having the voice of the archangel, but that hearing this voice is what wakes the dead)

As much as trinitarians like to point to particular translations of Hebrews 1:8, the next verse in the KJV says of Jesus "Thou hast loved righteousness, and hated iniquity; therefore God, even thy God, hath anointed thee with the oil of gladness above thy fellows."