Upgrading the game with DirectX 12 support is not going to magically increase the FPS. It will most likely result in performance improvements, but it probably won't be the huge improvement that some people seem to think.The thing is alot of us including me upgraded a few years ago to already a strong CPU mainly for this game. I bought a 6700k for about 400 euros 2 years ago and remember being disappointed when the fps didn't improve at al in big parks. I'm not gonna upgrade again to a 9900k which costs about 550 euros for something that probably won't make much of a difference either.
We are ignoring the problem. The problem is no DX12 support. You can upgrade all you want, run software all you want, install drivers all you want but if you don't grab the problem by the source it won't ever improve.
However, DirectX 11 is indeed the main bottleneck of the current performance issues when it comes to FPS; DirectX 11 can only use a single thread for its DirectDraw calls and since Planet Coaster has an enormous load of DirectDraw calls to execute it is slowing down overall performance and FPS of the game. DirectX 12 is capable of multithreading its DirectDraw calls and this should in theory result in a performance improvement.
Because of this the general advice is to use a CPU with a single core clock speed as high as possible, since higher speeds will benefit the single thread used by DirectX 11 DirectDraw calls.