Sims 3 64-bit Keeps Crashing and Lagging 2021

@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.

MacBook Air (13-inch, Early 2015)

2,2 GHz Intel Core i7 Dual-Core

8 GB 1600 MHz DDR3

Intel HD Graphics 6000 1536 MB

I have World Adventures, Ambitions, Late Night, Pets, Supernatural, Generations, High-end Loft, Outdoor living and Town life installed.

@CherryCherrys  To be honest, I'm a little surprised you're able to play at all with Pets—many players with your graphics chip have had to uninstall the pack altogether.  In your position, I'd hesitate to add Seasons, Island Paradise, or Into the Future, or really any expansion at all if you're seeing lag in general.  It's a little harder to say with crashing alone because that can happen for a variety of reasons, some of which have nothing to do with hardware.

You can try adding University Life and seeing how the game plays; like I said, it shouldn't break things if you're generally having a good experience.  After that, you can see where you are and make the decision from there.

For the crashing, save often, and use "save as" to rename the save so you have lots of backups.  (You can put a number on the end of the save to keep track, e.g. Bridgeport 5, Bridgeport 6, etc.)  If your game crashes and keeps doing so, trash the newest save and revert to the next newest one.  You can delete the oldest backups when you're sure you won't need them.

Hahaha after reading some problems I was worried too 🙌  I don't know if it's the world I play that helps, I play in Midnight Hollow, and another save in Riverview, I use the grafics on the minimum except for Sims Details and Textures that are on max and Mirror Reflexes Along With Water that are medium I guess.I'm not having lag but I use Nraas mods to clean my game, reduce taxis and animals on the streets, I noticed that other families in my world have pets but they look okay, the one invisible was the cat that I moved in with my sim's husband, when the husband lived alone with the cat and I would visit the cat could be seen. 🤔 

@CherryCherrys  Good to hear, but you might still have issues at some point.  I'd try changing their collars or any other accessories: use every cc option you have at least once, and see whether any of them makes a cat disappear.