[T]he term [Theotokos] while true in one sense lends itself to many wrong conclusions and deductions.
So does the phrase, Jesus is a man. Both term and phrase are meditations on the
mystery of the Incarnation. The Incarnation is a mystery not because we can't discuss aspects of it, but because we can never fully comprehend it. At the end of the day, all our articulations of the Incarnation will be inadequate.
My point is: terms which imply Mary is deity are error. Heresy often arises out of such speculation beyond the bounds of Scripture.
There is indeed an element of mystery regarding the incarnation and the nature of the Trinity. That is not license to add new Marian mysteries not based in Scripture.