Why not dismiss the trinitarian belief outright

[GUBU]

Well, for example, Marcion of Sinope, the inventor of the first christian canon and one of the earliest christians (he lived in the first half of the second century, right at the birth of christianity), believed that there were two gods, an evil one called yhwh who was depicted in the jewish scriptures and a good, hidden, god who sent Yeshua as his messenger.  He also denied that Yeshua had an actual physical body, that what the disciples saw was simply an image.

That's a big disagreement on christian fundamentals from what is now considered canonical.

By 'NT community', I mean the Early Church community portrayed in the NT. All sorts of exciting heresies developed towards the end of C1, but that's not the people I'm talking about. There is no credible evidence that I'm aware of that the community run by the disciples/Paul disagreed on the oneness of God.

As I said above, the first christians appeared in the second century CE. And given that we are basing our ideas on the earliest communities on writings from the 4th century, we actually know very little about them.