Two things here: 1) now that live preview settings are turned off, the monitor doesn't adjust unless you do something manually. So it is going to be as bright as you have it set and has nothing to do with how the camera is perceiving the scene. 2) you are also using a DSLR to focus via live view. By nature, DSLRs (with maybe the exceptions being 1dx mark iii and d780, i'm not sure about 5D4) do not have proper live view focusing as this isn't how they were really designed to be used so the overall focus performance is going to be worse. Again though, it has nothing to do with the monitor. the monitor is simply a larger screen, showing the same feed your LCD shows, it does not impact the camera's performance.