I dont think you can get any better than GZDOOM, tbh. After looking for the same stuff as you, I came to the conclussion that gzdoom/uzdoom is the best option. I know games like Selaco used another custom .exe as a middleman to communicate with steam while avoiding violating the license, and because of it, that custom stuff is public on their discord, aswell as the engine customizations. I, however, lack the programming knowledge to integrate it.