@YourMerigold The Public setting in Windows Firewall is actually more restrictive: it means you're setting your system as on a public network, and additional protections come into play. So leave that as it is.
For the issue itself, do you have Controlled Folder Access running? If so, please either disable it or set these exceptions for Sims 4:
https://crinrict.com/blog/2019/03/windows-10-defender-controlled-folder-access.html
If you see anything that mentions the EA App, be sure to give it access as well. I know you checked Windows Defender's settings already, but I want to make sure everything gets covered.
If changing the settings doesn't help, try disabling Controlled Folder Access entirely, at least long enough to test.
And please double-check that you do not in fact have an antivirus running in the background. Many computers come with one pre-installed even if the user hasn't paid for any particular service.