Does Isaian monotheism contradict the Trinity? One way of assessing that question is to consider the alternatives. From a unitarian perspective, who does Isaiah single out as the one true God?
Yes, Isaiah says “Yahweh” is the only true God. But who is Yahweh? Who (or what) does that designation stand for?
In principle, there are varieties of unitarianism (unitarianisms) which nominate opposing candidates for that singular distinction.
A unitarian could be a modalist. He denies the deity of the Father, Son, and Spirit. They are temporary projections of the one true God, who stands behind them.
A unitarian could be a process modalist. He could believe the Father, Son, and Spirit represent successive stages in the evolution of God.
A unitarian could believe that Jesus is the real God, while the Father and the Spirit are creatures or agents of the Son.
Or a unitarian might believe the Spirit is the real God.
More conventionally, a unitarian might believe the Father is the real God.
Does a unitarian reading of Isaiah favor one competing candidate over another? By definition, unitarians don’t believe Isaiah uses Trinitarian categories. That would be anachronistic.
So Isaiah doesn’t single out the deity of the Father, to the exclusion of the Son, or vice versa. From a unitarian perspective, he doesn’t employ that framework even for purposes of contrast. It’s not “God the Father” is the true God,” while Jesus is his agent. For that would still be framing the issue in Trinitarian nomenclature, even to oppose the Trinity.