Trivially irrelevant. Not least because the same chapter involves the Spirit bearing witness of Jesus (so not personally Jesus, no more than the blood and the water are personally Jesus though Jesus came in water and in blood), and yet on par with God bearing witness that He has borne witness concerning His Son. But the Trinity is implied elsewhere in the scriptures, in ways and terms not much different in meaning from the spurious Johannine comma. After all, even the comma doesn’t use the term Trinity (or even the term “Son”), and yet you recognize that’s what it’s talking about: three distinct Persons in heaven that are not merely “one” in personal cooperation!
(I’m always amused when non-trinitarians bring up the comma acknowledging it’s talking about the Trinity: they would be more consistent if they interpreted it as non-trinitarian as well as spurious.)
Doxologies are on the same par as prayers, and doxologies including the Holy Spirit as well as the Persons of the Father and the Son happen a lot.
But your observation is pointless, because you wouldn’t count any such evidence even if you saw it: you would just reckon the prayer was praying to the Son or to the Father and calling one or the other “the Holy Spirit” or similar titles. And even if the Christians praying to the Holy Spirit included prayer to the Father and to the Son as well, that wouldn’t be any different in principle than a doxology to the Father, Son, and Spirit, or baptising in the name of the Father of the Son and of the Holy Spirit, or any other religious reference to the Father and to the Son (as distinct persons) plus the Holy Spirit. And you know those happen with some frequency across many types of NT scripture, so whatever ways you disregard the Spirit as being a third distinct Person there you would apply to the prayer per se, too.
We can test that theory if you want: a number of the doxologies are prayers calling upon Jesus and God along with the Holy Spirit to bless people in various ways. You don’t think those count as prayers to the Holy Spirit even though they’re prayers to the Father and to the Son as distinct persons, because…?
(If it comes to that, your observation is pointless in another way, too, because you acknowledge people pray to Jesus, even as God, but you deny Jesus is the self-existent God Most High. Other unitarians take much the same evidence, along with the personal distinctions of the Spirit which you don’t recognize, and conclude the Holy Spirit is another high-ranking creature like Jesus, maybe the second creature created by God.)
St. Paul in 2 Thess 3:5 prays that “the Lord” may direct the hearts of his readers “into the love of God and into the steadfastness of Christ”. The Greek syntax certainly indicates two distinct persons for God and Christ; but it would be very unusual stylistically for St. Paul to redundantly refer to one person directing believers to that same person (i.e. that the Father may direct our hearts to love of the Father, or that Christ may direct our hearts to the steadfastness of Christ.)
In 2 Cor 13:14, Paul blesses his readers with the salutation, “The grace of the Lord Jesus Christ, and the love of God, and the fellowship of the Holy Spirit, be with you all.”
In the doxology of Eph 1:2-14, the Father is praised for choosing us (3-6), the Son is praised for dying for us (7-12), and the Spirit is praised for sealing us (13-14).
Rom 1:1-4; Rom 8:9-17: each set of verses distinguishes not only the Son in comparison to the Father (or God) but also the Spirit.
2 Cor 1:21-22: ditto (although admittedly more vaguely)
Heb 9:14: ditto
1 Pet 3:18-19 (if the grammar reads “in whom”); 4:14: ditto
In all these cases the grammar and/or the context indicates that three (not only two) persons are being spoken of.
Now, I’m not going to say that there is as much evidence about the distinct personhood of the Holy Spirit as there is even about the most-high deity of the Son (of which there is a ton). The scriptures talk a lot about the Spirit of God or the Holy Spirit or other names of that sort and usually those can be identified simply enough as God in such a way that a unitarian (or a modalist for that matter) could just call them a reference to God the Father (even though some unitarians recognize the personal distinction of the HS and so don’t identify Him with God Most High). And when it seems obvious the Spirit of Christ is being talked about, someone might say that’s only Jesus personally.
But I think there’s enough testimony to indicate, as you yourself agree in your own way, that the Spirit is singular (not two spirits one of the Father and one of the Son) and yet also the (singular) personal spirit of two distinct persons (the Father and the Son).
And I think there’s enough testimony to indicate that the spirit of the Father is God Most High – as again you yourself agree in your own way.
Putting those two ideas together, though, gives us a single personal spirit of God Most High, Who alone should be religiously worshiped, which is the spirit of two distinct persons, Father and Son, with the Son being personally and religiously worshiped along with the Father, with at least the Father clearly being God Most High Who alone should be religious worshiped – the religious worship of any lesser lord or god being idolatry.
That isn’t trinitarianism yet, but it’s a position which someone cannot consistently hold while also complaining about the idea of God being a single deity of multiple persons. Which thus opens up at least binitarianism (two distinct persons of God, Father and Son) without even going into high Christology evidence about Jesus specifically!
And once that composite position is held, the only step from it to trinitarianism would be acknowledging and accepting evidence of the personal distinction of the Spirit with the persons of the Father and the Son. For which there is some weighty scriptural evidence: enough so that some unitarians take the route of the Spirit being a distinct person who thus cannot be God Most High regardless of what the scriptures indicate along that line instead.