I’m sure it’s probably been said, or 2 seconds googling would find: there’s no fullscreen exclusive mode in DX12. The reason being they can get the same if not better performance borderless windowed.
Happily the DX12 patch is working well for me; it’s considerably smoother and looks clearer - not that it’s better optimised, best guess is it has better support for features that already exist, it just works properly now.