KiCad is a beautiful EDA tool ( schematic capture, pcb layout, …), it is also free.
the developers intend to include a simulator in the program in their roadmap, but it is a far future.
Eeschema (KiCad’s schematic editor) is able to export spice netlists to do simulations directly, you can also associate the directives as a text starting with +pspice, you can find more in Eeschema documentation.

below  we have two diodes in series as the blocking capability of one (3kv) is not enough for a source of 5kv, the problem is the two devices are not identical, connecting them in series making them have the same leakage current and hence different reverse voltage sharing, to solve that we put two resistors as shown.
assuming equal resistors of 100k ohm, the result is shown, there still some voltage difference but not that much(500v lol :D).