Yeah, that's the solution I'm thinking about, so far games that I had developed in the past, in fullscreen, changed the desktop resolution to match the game. But, the other way around seems to be the best solution.
However, I simply cannot allow the game to just upscale, because it may get heavier the higher your desktop resolution is, for instance, if you have a potato GPU but a 4k monitor, the game will run like crap, so the solution I have is to run the game in a smaller res and just upscale at the end after all rendering is done.