February 2021
After managing to successfully install all expansions and stuff packs, I tried to add my cc and mods. These were all just copied over from my previous computer, where they all worked perfectly fine. The installer worked fine for me, but once I tried to open the game it just wouldn't even go past the first black loading screen into the game and said it was not responding. I tried restarting origin, but this didn't work.
February 2021
@AddyAva Try downloading fresh copies of your mods and custom content. There are issues with copying over some files, mostly or entirely Sims3Packs and the bundled .dbc (cc) and .ebc (store stuff) files that result from installing them. But it's best in this kind of situation to download new versions of everything and add the content slowly, testing each time to make sure the game still runs.
Start by moving your entire Sims 3 folder out of Documents > Electronic Arts and onto your desktop. Open the launcher to create a clean folder, make sure the game runs, and then download some content and see how it goes.