The first rider cannot be Jesus. Revelation 6 is describing the beginning of the tribulation, and Jesus is supposed to come back to end the tribulation
Matthew 24:29-30
29 Immediately after the tribulation of those days:
‘The sun will be darkened,
and the moon will not give its light;
the stars will fall from the sky,
and the powers of the heavens will be shaken.’b
30 At that time the sign of the Son of Man will appear in heaven, and all the tribes of the earth will mourn. They will see the Son of Man coming on the clouds of heaven, with power and great glory.c).
In revelation 6 the Lamb is the one that opens the seals, as said in earlier comments, Jesus cannot at the same time be the one riding the white horse.
Also, when Jesus comes, at the end of the tribulation, he will not be the only one riding a white horse (not the one in chapter 6). He will bring others from Heaven with him who also will be on white horses. We do not know how many of these white horses there will be.
Revelation 19
11 I saw heaven standing open and there before me was a white horse, whose rider is called Faithful and True. With justice he judges and wages war. 12 His eyes are like blazing fire, and on his head are many crowns.He has a name written on him that no one knows but he himself. 13 He is dressed in a robe dipped in blood, and his name is the Word of God.14 The armies of heaven were following him, riding on white horses and dressed in fine linen, white and clean. 15 Coming out of his mouth is a sharp sword with which to strike down the nations. “He will rule them with an iron scepter.”[a] He treads the winepress of the fury of the wrath of God Almighty.
Here is a description of Armageddon, which is not the same as revelation 6. In revelation 6 it is the beginning of tribulation with only one white horse, and in revelation 19 it is the end of the tribulation with more white horses.
I think Jesus cannot be the horseman of revelation 6 as how WT describes it.
If they say it is Jesus, then revelation 6 is his second coming, and revelation 19 (in correspondance with the text of Matthew 24) is his third coming???