I am not sure what is the object distance. If it makes the simulation too large, you may do this in 3 steps:
1: using the source and the obstacle to do the first simulation, project the monitor data on to the front of the metalens, using farfield3D, including E and H:
https://optics.ansys.com/hc/en-us/articles/360034930733-farfieldexact3d-Script-command
2: using the projected EH data as import source, simulating the metalens. Using monitor data to define the spatial field profile of a source in FDTD
3: project the monitor data on to the inmage plane.
the two simulation files should be small, compared to simulate the whole process from object (the obstacle) to the image plane.
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