That is overly vague. Which of his teachings are followed in order to constitute "Thomism?" For example, if I agree with Thomas that the bible is the divinely inspired word of God, is that Thomism? What if I agree with him that God is Father, Son, and Holy Spirit?
Thomas did not contribute any novel speculations on this matter. He did repeat Augustine's psychological analogy of the Trinity, that the Father is like a mind, the Son his self-conception, and the Spirit the love between the Father and the Son. I don't think it's great to take that as a factual, scriptural explanation of the Trinity, because the bible doesn't teach anything like that, but it's Augustine, not Thomas.
The medieval school committed to using metaphysics to speculate on the inner workings of the divine nature was chiefly Scotism, not Thomism.
I think we would all agree that there are areas that are not open to our investigation. But it would be more helpful to state a specific proposal as to what areas cannot be investigated, and how Thomas transgressed that, and in order to do that you'd have to read Thomas.
I have actually been fairly critical of the label "Reformed Thomist," because I don't think it's an accurate reflection of the primary theological influences on the Reformed tradition, and I disagree with Thomas on points like the authority of the church vs that of Scripture, justification, and the sacraments.
But it's not helpful to label sections of the Reformed Church, like R.C. Sproul, as "Thomist," and then write them off. It would be about as fair as me writing off Lane Tipton as a "kantian" because Van Til appropriated the transcendental argument from Kant.