Before opening your main save
Back up your Saves, Tray, and Mods folders. This gives you a safe copy before testing anything after a patch.
Check if mods were disabled
After many updates, The Sims 4 may turn off custom content and script mods. Re-enable them in settings, restart, then test again.
Update gameplay mods first
Gameplay mods, UI mods, script mods, traits, aspirations, careers, and tuning mods are more likely to break after a patch than simple CAS items.
Use a test save
Do not use your most important save for testing. Make a new temporary save and check whether the game loads cleanly.
Patch day rule: If you are not sure a mod is updated, leave it out until the creator confirms it is compatible.
Test in batches
Add your mods back in smaller groups. If a group breaks the game, split that group again until you find the exact file.