While he could do that, nothing we have seen from Strange suggest he could do this that quickly. Or he would have done it to Thanos
Thanos has the space and reality Stones.
No way mirror dimension would stick.
We don't know if the stones work outside of their own universe.
They don't.
Someone mentioned "Dormammu" which happens to be outside the universe, doesn't it? The fact that it worked there means it could work in other universes, right?
His universe is still a part of the MCU universe. ---