pprisk It does sound like the issue is your old save, but that doesn't mean the entire save is unusable going forward. You can try emptying out your sims' inventories, and the household inventory as well; you can move your sims to a different lot; you can play a different household and come back to your original one. These are all tests to see whether you can isolate the cause of the issue. If something works but you don't like the results, let me know—there might be a less-intrusive fix as well.
As a side note, your computer could probably take a standalone graphics card. I don't know what power supply you have, but a used GTX 1050/1050 ti/1650 would be cheap and fit in most cases, and many models don't require a supplemental power connector. All of these are capable of running Sims 4 on ultra graphics settings. If you're interested in upgrading and want to talk more about it, let me know.