@MollyRose95
Is your Mac on a soft surface? It looks like it in the pictures and it will dangerously overheat if so. The OS will throttle power to the game, resulting in poor performance including lagging, crashing, glitching graphics and even shutting down of your Mac to prevent hardware damage. Make sure it is on a hard surface, preferably elevated a little, so air can circulate underneath. Your Mac has an integrated graphics chip which isn't designed for gaming, it will always struggle more than a discrete/dedicated GPU. Add excessive heat into the mix and it will perform really badly.
Can you attach your Config.log file to a post here? It's in the Sims 4 folder. This is to check if your Intel chip is supported in the game file and at what maximum setting.
Could you also please attachet a screenshot of your in-game graphics options, making sure it also shows the laptop mode toggle at the bottom of the settings.
I've seen a couple of other reports of users with a similar issue on your model Mac. Not sure if they fixed it or not but I'll try and find them to merge together.
Does this happen in windowed mode too?