The root of all of the difficulty, debate, and complexity of the doctrine of the Trinity is the conclusion that three distinct entities are represented as Almighty God in the Bible. This conclusion is apparently incoherent, because the Bible teaches clearly that there is one God. Therefore, immediately, work begins to understand what exactly is being said by the Biblical authors, because the principal of charitable interpretation seeks to give authors the benefit of the doubt, and not just discard them as teaching obvious contradictions, if what they say can be reasonably understood another way.
It is the position of Unitarian Christians that the Bible does not actually teach that there are three distinct entities who are taught to be Almighty God, and rather, that the one God is explicitly presented as the Father alone (1 Timothy 2:5: 1 Corinthians 8:5-6, Ephesians 4:4-6, John 17:3). Unitarians, then, hold that each of the texts which others assert are teaching that entities other than the Father are Almighty God in reality do not teach that, and can be understood in different ways. This offers what is in comparison, a very straightforward view of God, with yes, many exalted statements of His Christ, but ultimately, each of these is comparatively easy to understand within a Unitarian framework, and the immense complexity of Trinitarian speculations is therefore avoided.
So, the complexity with respect to God's ontology in mainstream Christianity comes from concluding that the Bible presents what is essentially a puzzle - an apparent conundrum that there are three who are Almighty God, who are not each other, and yet, there is one God. Mountains of literature are then written in attempt to resolve the apparent conundrum. But, stepping back, one should consider whether all of the complexity is actually coming from a mistaken supposition somewhere along the way.
Consider someone who has concluded or assumed that the planets and stars orbit the earth, rather than all heavenly bodies orbiting others based on gravitational attraction. Then, they set out to explain or model exactly what is happening in these orbits. Very quickly, the model gets extremely complex. The mathematicians and astronomers assigned to the task may devise ingenious ways to make the model fit observations. However, the complexity comes from a mistaken starting position, which was actually not true.
In the same way, the Bible never actually presents God as a conundrum, nor do the authors of Scripture give any hints that they felt the pressure to explain what would be a clear apparent contradiction in a major tenet of their theology. There is simply nothing in Scripture which would suggest - "God is one in three, and we cannot explain how". Instead, everywhere in Scripture, it seems that the authors simply assume that others will know what they are referring to when they say "God", and speak of Him as a single subject. So, they never feel a need to explain it, nor do they give any hints that they believe their doctrine of God to be a mystery, or something which the average or uneducated person would have any difficulty with. For these authors, there is "one God", who is the one Christians call "Father". That claim is simple, and so does not bear explaining, only stating - and it is explicitly stated verbatim (1 Corinthians 8:5-6, Ephesians 4:4-6).
What advice would someone give to the geocentric scientists referred to earlier, as their calculations get ever more complicated, and they pour ever more fervor into justifying their starting conclusion? Most would advise them to step back, and to reconsider whether their starting position was incorrect, and whether this incorrectness is the source of all of the complexity. Trinitarian scholars have devised ingenious ways of accounting for all of the Biblical evidence, given their presuppositions. And yet, it still remains true that one can go to the average church attendee of any denomination which holds to some form of the doctrine, and that person will be utterly unable to explain how exactly they believe that the three Persons are one God. If they attempt an answer, the more learned among them will usually quickly condemn it as heretical in some form or fashion, and yet, with even the most sophisticated and intricate formulations, the Biblical tension is never fully resolved, nor is the intellectual tension of three being one, and yet distinct, ever finally resolved. It instead remains a constant source of paradox, and confusion. And yet for all of this, the Bible is silent in so much as describing the doctrine even one time (something like, "God is three in one"), let alone explaining it, or submitting it as a mystery.
The doctrine of the Trinity, in its many forms, is, from a Unitarian perspective, a result of some brilliant and influential people coming to the Bible, misunderstanding its teaching about who Jesus is, and then working diligently to attempt to reconcile that misunderstanding with the other things that the Bible teaches. But, their initial conclusion was incorrect. Therefore, the interminable issues and complexities associated with the doctrine were inevitable, because these things arose from a faulty foundation of claims which actually cannot be Biblically reconciled.