Post-DDU, I've only tried opening the Sims 4 in windowed mode. I don't know how to take it out of windowed mode. Plus, I typically play in windowed mode, so I do have to make sure it can run that way.
I checked my Steam games and they're still showing the "Input Signal Out of Range" error too. So it's not just Sims 4 having an issue, post-DDU.
Unfortunately I don't have the ability to follow your advice with the cables right now. In order to get at them, some heavy furniture would have to be moved, I'm severely disabled, and have no one to help me move it. If it's absolutely necessary, I can save up some money to eventually hire someone to help me move it, and then I could access the cables. But of course, I would rather save that as a last resort. What I can tell you in the meantime is that I had all the cords and everything else hooked up by professionals, plus I have the computer, monitor, and all other parts regularly cleaned with compressed air. I've used this monitor with this computer for four years, and played the Sims 4 (plus many other games) on it hundreds of times without issue. So I find it unlikely for the problem to lie there. I don't have access to any other monitors or cables besides the ones I'm currently using.
I have thought of a couple more things that could give clues as to what's happening.
First, when I was custom re-installing the driver yesterday, it asked me if I wanted to do a "regular install" or a "clean install." I didn't know which one to pick, but I went with "clean install." Was that the correct choice? If not, perhaps that's why the issue persists?
Second, I just realized that the problem started soon after reinstalling my Windows 10 operating system. For some reason, my Windows 10 OS gets corrupted a lot – about every 1.5 years. So every 18 months or so, I end up having to load all my files onto an external drive, press the "Reset this PC" button, choose the "remove everything and perform a clean install" option, then reinstall all my programs and load my files back on once it's done. The last time I performed this was about a week ago and, for some reason, it didn't go the way it usually does. I did everything the same way I always do, including the "remove everything and perform a clean install" option. Yet, when the new Windows 10 install was complete, all my files and some of my programs were still on my computer. So I downloaded the few things that seemed to be missing and carried on with my day. I played Sims 4 without issue, as well as Stardew Valley. About 2 days later, a Windows pop-up came up saying something like "Error: Windows could not find any drivers on this computer." (I wonder how it was able to run those games, then, if there were no drivers?) I went to GeForce, downloaded their app, and went through the automated process of installing the newest driver. It was after this that the problems started. I have not been able to successfully run Sims 4 since. I got Stardew Valley to run one time after this, but no more. Due to the timing, I feel like something happened then that is causing the issue now, but I'm not sure what or how to fix it. Should I try resetting the PC again? And what is the proper process of installing drivers after a PC reset, considering my computer was fine running games without (allegedly) any drivers at all, but hiccups now that it has a driver installed?
By the way, I did two full scans – one before resetting my PC and one after – to check for viruses and malware, so neither of those are an issue.