You do not need to simulate so long in z axis, since it is in the uniform material. What you should do is, place the monitor close to the lens which significantly reduces the simulation volume and simulation time. Then you do one farfieldexact projection along z, to find where the image plane is. Usually we define the focal plane at the max axial intensity. Once you get this location, use it to do a second farfieldexact calculation for this plane so you will know the image spot.
Please refer to the metalens example https://optics.ansys.com/hc/en-us/articles/360042097313-Small-Scale-Metalens-Field-Propagation
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