The Masoretic consonantal text is beyond dispute: וְהִבִּ֣יטוּ אֵלַ֔י אֵ֥ת אֲשֶׁר־דָּקָ֖רוּ, “they shall look unto me, ’ēt whom they pierced.”
Yes of course. Yahweh is pierced/stabbed through. That is true whether you prefer 'the one' or 'me' or as some translations both. Yahweh is the referent. This expression however was objectionable to many 2nd temple Jews, hence we see the LXX and Targum revisions.
The second half of the verse has inspired volumes of interpretive conjecture. As traditionally rendered, the people mourn/grieve over a person, "him". There is no other 'him' introduced than Yahweh, so Is the "him" Yahweh? this make the verse rather awkwardly phrased, changing from Yahweh speaking, to a third person mid-thought. The rendering:
And I will pour out on the house of David and the inhabitants of Jerusalem a spirit of grace and supplication. They will look on me, the one they have pierced, And they have mourned over it, like/as a mourning over the only one, and they have been in bitterness for/because of it, Like a bitterness over the first-born.
is both accurate and attractive as it resolves any awkwardness. But as you know that is not how many readers took it. By reading the pronoun literally masculine, because of the metaphor that follows, a whole slew of interpretations ensued, as is seen in the Rabbinic, Targums and Christian writings.