@CherryCherrys There are a few ways to find bad custom content, but the gold standard is Method by Mell, described here:
https://sims3.crinrict.com/en/2011/01/tutorial-how-to-get-rid-of-unwanted-custom-content.html
If you look through the cc in each .dbc file separately, you may find something that doesn't belong, in which case you can uninstall it through the launcher. If you don't see anything suspicious, it's possible that some cc you actually want is the issue, and it gets a bit more complicated.
If you still had the invisible cat, you could load the game with each .dbc file separately (same Method by Mell, but you're loading your save) and see which one makes the cat invisible. From there, you might have to guess at which cc item was the issue, although pet accessories would be an easy first suspect.
Since you don't have the cat anymore, you might try experimenting on a new save, with a cat included, or you could wait until another pet becomes invisible and experiment then.
Without knowing what hardware you have, it's difficult to say how your Mac might handle those packs, although University Life is probably fine. If you'd like to post your specs, please click the Apple icon in the upper left corner of your screen, select About This Mac, and list everything except for the serial number.
Seasons is quite demanding but doesn't seem to cause many performance issues in this version of Sims 3. The graphics issue with roads in winter is annoying, but it doesn't seem to cause crashing. The world of Isla Paradiso runs poorly on any machine, but the pack's contents are fine, also demanding but not as much as Seasons.
If your game runs fine without any custom content, you could add one pack at a time and see how it goes. If it lags with no cc and no mods in a new save, it's probably not a good idea to add demanding expansions. But again, University Life has only a moderate impact and doesn't seem to be much of an issue, so if you really want one more pack, it wouldn't be a bad choice.
