Dimmed Screen

@ZoomerGirl  Your dxdiag shows a number of Windows errors that are worth addressing first.  So please run a couple of basic checks of your Windows system files:

• Hit Windows key-X
• Choose either “PowerShell (Administrator)” or “Windows Terminal (Administrator),” whichever option is offered
• Inside the window that appears, copy and paste “DISM.exe /Online /Cleanup-image /Restorehealth” without quotes, and enter
• The system will start validating soon. If it throws an error, please list it here
• After it reaches 100%, hit Windows key-X again
• Again, choose “PowerShell (Administrator)” or “Windows Terminal (Administrator)”
• Inside the window, copy and paste “sfc /scannow” without quotes, and enter
• Post the message you receive here

Restart your computer, hit Windows key-i, select Update & Security, and click the box to check for updates.  If any install, restart again afterwards.

For Sims 4 itself, try disabling Game Mode and Game Bar, both found under Windows settings > Gaming.  (And then restart yoru computer.)  If that doesn't help, let me know whether using "windowed fullscreen" makes a difference.

If the game keeps reverting to fullscreen mode, try moving options.ini out of Documents > Electronic Arts > The Sims 4 and onto your desktop.  This file stores all your game options, so you'll need to redo other settings as well, but if the reason your settings rever is that this file is corrupt, forcing the game to spawn a new one is the only fix.