For EM simulation, the open source ones that are most accessible are openEMS, and FastHenry2 + FaserCap. Geometry description in FH can be a pain if the design is complicated. openEMS needs processing of the GDS files to extract the polygons. KLayout does a good job here. It has a CSG engine that is easy to work with.
Here's a sample setup file that should get you started. https://github.com/diadatp/sky130_rf_tools/blob/main/openems/main.m
Be prepared to wait a long time for simulations to run. Especially if you set the correct boundary conditions.