Ran a DISM scan on my PC, restarted and the game now works. No idea what the issue was, but I'm not complaining.
Thanks to all who suggested fixes.
@RegularEggKing What antivirus software are you using? This does sound like an antivirus issue, and a few of the programs now require extra steps aside from simply adding an exception for Sims 4.
@RegularEggKing Please make sure you've adjusted all of the Avast settings described in this article:
https://crinrict.com/blog/2020/10/add-exceptions-to-anti-virus-programs.html
If that doesn't help, move your entire Sims 4 folder out of Documents\Electronic Arts and onto your desktop, and see whether the game opens. If it does, a new folder will be created with no content. Don't add anything to it yet; just see whether you can get far enough to start a new save and play in live mode for a bit.
If that doesn't work either, please create a new admin Windows account on your computer and try to play in that. Make it a local account, as in, don't sign into Microsoft or any other services. You'll be able to login to your current Origin account and shouldn't need to redownload anything.
