DX12 certainly improves performance, at least on my machine, even though officially it isn't supported. And it's well established that the Livery Editor will crash if run under DX12. My question is: will a created livery crash the game if used in a route? Or, once created, is it just another livery file? In other words, would it work to create a livery under DX11, then restart under DX12 to run?
In my experience, unfortunately the answer is that the game will crash if any custom liveries are used.
The FGW mod does work under DX12 because it is a livery (officially unofficially) made by DTG, as opposed to a Livery Designer creation.