Sims 3 64-bit crashes when loading

@The13Kitty  That strongly suggests that the crashing was caused by something in your old Sims 3 folder, whether the issue is the save itself or some other content.  What you can do now is copy over the save, nothing else, and see how it runs.  Each save has its own folder inside the Saves folder, and you can simply right-click to see the Copy option and then right-click the blank space inside the destination folder for Paste.  If the save takes place in a Store or custom world, install that world too, but nothing else.  Since you're copying your save rather than simply moving it, you don't need to worry about any missing content: the original will remain intact while you experiment on the new copy.

If the save runs fine, you can transfer over the rest of the content you want to keep, but leave behind cache files and anything else that will get regenerated as needed.  Here's an explanation of what the files are:

https://sims3.crinrict.com/en/2011/01/faq-user-files.html

Copy them over in batches, testing each time to make sure the game still works.  If one batch of files causes another crash, you'll need to split it and retest until you find the culprit.

If one of your old saves won't load in the new folder, things get a bit more complicated, but you still have a couple of options.  Let me know if this is the case.