@dreamystarbi When you uninstall Sims 4 itself, through the EA App, you remove the game but (usually, under normal circumstances) not your saves or other user data. Those are stored in Documents > Electronic Arts > The Sims 4, and it's best to make a backup of this folder or its critical contents, e.g. the saves folder, just to be on the safe side. As for your DLC, your account owns it, not your computer, and it will be available to reinstall whenever you want.
This is not only about uninstalling the game but also in case you run into computer trouble. You can upload the data to a free cloud storage service like Google Drive, or store it on an external drive or USB stick. In either case, I'd suggest zipping the folder(s) first both to save space and to lessen the chance of corrupting any files.
The Sims 4 base game takes up about 20 GB storage, with packs ranging from negligible in size to maybe 3 GB for a couple of expansions. If you're seeing more of an impact than that, it's possible you have a second game install somewhere. Let me know if you need help finding it.
The EA App is relatively small, so removing it doesn't make much of a difference. But it's also not harmful to do so as long as you remember to reinstall it when needed.