I am hoping someone here has a fix for an issue that is plaguing my 5yr old daughter...

My wife introduced her to sims 4 about 6 months back with both of them playing it together and my daughter loved it. She begged for her own computer for Christmas and she got it, all loaded up with Sims 4 and a bunch of the addons (I apologize as sims isn't something I know much about... I just am the IT Dad, so if I get terminology wrong and it makes a diff please ask for clarification). Since she is 5 when the computer gets turned on, one of us will login to the computer, open up EA App put it in offline mode and then disconnect the wireless so it gets updates but we don't have to worry about her online.

After the 1st 2 months she started complaining that the sims take forever to load and save... so I had her show me and sure enough the load screen would take like 20-30min to load... So I sit down to take a look and realize her Sims 4 folder in My Docs is over 20GB. I move it (so I dont delete her sims) and she starts fresh... she threw a fit for about 10min and then was fine (but still tells everyone I killed her sims). A month or so later I decided to check it again, 15GB so I did the same thing... I checked less than 2 months later... she is at 15GB again.

So I looked into what was there today... she probably has created close to a thousand sims (on the plus side, she is able to type better than she can write and is actually learning how to spell names). Is there anything that can be done when we are talking this many sims? I really don't want to go another round of being the sim killer again... I have no issues with mods, especially if they improve performance for something like this. Her computer has an NVME and the game works great, until there are too many sims. She says it works fine one day, she goes to play the next and its broken again... like it hits a specific number of sims and that final one kills it.

FPS boosting and optimization can't really help much if you're trying to run hundreds of sims in a single save. You could use the in-game settings to decrease max sim count per-save and thus increase automatic culling, but that's still removing sims.
Add-ons for the sims are referred to as mods. You may want to check the mod list and make sure it's not excessive, as that will also lag the game (especially loading screens).

You have to tell her she can't make new Sims all the time, as the game itself has a limit (200 max), otherwise the game will lag badly.
Also, when the pages load, is there a little box that says "mods" in-game, and there's a small box at the bottom you can check to turn that off? If so, check the box you don't want the list to show, as it also slows your game. If she has mods, your wife provided her, make sure they are all compatible as well. Sometimes they don't get updated, and the new updates mess them up.