@bi0hzrd I read through this thread and the one at NRaas again in case I missed anything, and I have a couple questions and suggestions that might feel redundant to you, but I want to make absolutely sure I'm not overlooking any possible sources of error. So please bear with me.
As I mentioned before, a lot of the longer loading time for a new save is due to the game trying to place the EP lots in the world. (Part is probably creating WorldCache data, but I'm not sure how much.) As you can imagine, this takes some calculating. I don't know why the game would be calculating this again in an existing save; that's not standard behavior and I've never seen it do so myself, at least as far as I can remember. But the fundamental problem very much seems to be a failure to read certain files, so not reading whatever data says "this process has already happened" is in line with everything else.
This reasoning of course only applies to existing saves, not new ones. The default behavior is that a new save should take extra time to load regardless of whether you've previously loaded that same world, again due to placing those EP lots. If you're seeing brand-new saves take the minimum amount of time to load, but only within a few hours of the last time you loaded that world, that's the oddity, although to be honest I haven't tested this myself. So I can't say that this is unique to you rather than a random glitch built into the game. I will test myself a little later.
A couple suggestions though: First, you can try running Sims 3 as an admin, as in, right-click the executable or its shortcut and select "Run as administrator." (Having an admin account allows this function but doesn't apply it by default.) Please also make sure that Windows Defender's Controlled Folder Access hasn't reenabled itself. Here's a guide in case it helps:
https://crinrict.com/blog/2019/03/windows-10-defender-controlled-folder-access.html
Please also let me know if you've moved the location of Documents, or created a symbolic link to redirect the Sims 3 user folder. Essentially, I'd like to know if the user folder is stored anywhere other than the default C:\Users\your username\Documents\Electronic Arts. And what kind of install do you currently have? I know you tried a few, all with the same problems; this is just in case I need to reference something platform-specific.
Your GPU can definitely handle the increased load of setting DSR to 1.78, i.e. rendering Sims 3 at 1440p, so there's no reason not to keep doing that if you're satisfied with the results.