If the trinity is such a fundamental doctrine, why doesn't the Bible...
This is the foundational problem that likely cannot ever be solved because we are talking to people who were exposed to the Watchtower religion (as I assume we all were), which was a cult that brainwashed people.
Not all of us move away from it at the same pace. It teaches suspicion, automatic hatred and prejudice (if not outright bigotry) for those who teach traditional beliefs, and instills individuals with a mistaken sense of a better understanding of academic scholarship than academics.
Unless you are an academic, your qualifications with religion is that you are good at getting tricked into joining a cult or having a hard time being convinced to leave the cult you were born into.
That is not a great qualification for any of us.
Worse, the Watchtower created and offered a paradox that some obviously still carry with them: that the world was literate, that Bibles were mass produced, and that from these original Christianity sprung forth before Christendom went nuts and invented the Trinity.
The opposite is true. Until World Wars, many people around the world could not read, even in the West. It was the famous barefoot walk of Mary Jones in Wales that spurred the birth of the Bible socities that made it first possible for common folk to own Bibles in the mid to late 1800s. (That's a long time to wait for yor 1st Bible since Jesus walked the earth.) These Bible societies helped stop the widespread illiteracy in the world via the coulporters who worked for them.
The Trinity is a dogma, not a doctrine. This is very important because a dogma states a religious truth that did not form from human reasoning or resources. A doctrine is often found via texts or can be explained, but a dogma cannot.
The Trinity had to be presented in a creed format which is a statement that a dogma is under attack by a heresy. A creed offers counter arguments to those presented by heretics, not reasons for believers to consider a dogma.
The New Testament Canon itself was an answer to a different heresy that begun in the 2nd century by Marcion of Sinope who claimed that salvation was limited to a select few with special knowledge who rejected the God of Abraham and embraced his own canonical collection. Marcion claimed Paul and Luke also rejected the God of the Jews, and that Jesus himself taught such a gospel. So the Church replied by creating the New Testament Canon which raised Luke among the Apostolic works and highlighted the Pauline epistles above the Petrine. This was not to create a source for doctrine, but to end the Marcionist movement. An entire work by Tertullian exists on this called "Against Marcion."
While one might indeed find texts indicative of the Trinity formula such as Matthew 28:19, the dogma of the Trinity came before Matthew could be officially cited as an authoritative text for Christian doctrine.
Not that anyone could or would, as people who were not scribes generally did not read or write. And until the mid to late 1800s, common people did not have the means to own a Bible.
Once they did, especially in America, it led to the foundation of the American Bible Society. At the same time another event in American history took advantage of the sudden surge in Bible distribution: the Second Great Awakening.
While there were some advantages to this period in the USA ( the spread of literacy and the Methodist religion to the West), it brought disaster to many like the Great Disappointment, the rise of Mormonism, and an attempting copycat of the great Foreign and American Bible Societies, Charles Taze Russell.
To try to fight what is ingrained about the Bible in so many is impossible. Why? Like the Trinity "mystery," it is more a feeling for many that somehow the old Watchtower formula of "Bible first" must be true. It's impossible and doesn't matter anyway because we live in a reality where to be Christian is, generally speaking, to be Trintarian, like it or not.
And that is the problem. Like holidays, Watchtower teaches people to dislike it, to hate it, like the way so exJWs hate Christmas carols even though they have not been a Witness for many years now.
You might be in the minority, and that is fine, but it won't make the majority incorrect. It doesn't work that way. We are not in the Watchtower anymore. If you want to be Christian and unitarian, that does exist. It's a small number, but that is why there is the word "unitarian."
But you don't change history in the face of your choice of beliefs. It's like my people, the Jews. Jews don't believe in Jesus at all. (I barely believe in my own people.) But Jesus was real. Resurrected? Maybe. Why not! Jews go one way, Christians another. It doesn't change history.
Just because we might not belive in the Trinity does not change the fact that the actual Christians that came from the Apostles are the Trinitarians. They were not led by the Devil or trying to twist things like the Watchtower teaches. This is their genuine, Apostolic faith.
Like Christmas carols to some exJWs, this will never, ever sit well for some. All the evidence in the world will not work. It's called an "emotional scar." This is where you blame the cult, not the Trinity.