This indicates that your FPGA simulator may not parallelize well. It may be written to only effectively use two threads.
If this is the case more cores will make (almost) no difference, it may even be negative as more cores tend to mean lower clock rates. For this type of application you should look for highest clock rate / best single thread performance and good thermals so you can use turbo boost as long as possible, for me this sounds more like a base 16".
Once a long time ago, I was responsible for procuring computer system for all type of simulations for a car OEM. In this situation we tried to understand what kind of computer architecture would give the best performance for a certain type of analysis. For instance FEM analysis required few very fast CPU because it parallelized very poorly. On the other hand CFD parallelized very well, so we could use simpler CPU but many many more of them.
Maybe you could research your application a bit to understand how it works and what would benefit you, in that way you may get better advice.
You may also want to google "Amdahls law" to understand a bit about how parallelization works, or sometimes does not work.