For Sims 1, I did not download any extra content for that series. I had everything I wanted from the expansion packs.
For Sims 2, I downloaded the College Adjuster (I forgot where I got it from) which I still use today. I use it to change when the final exam for my Young Adult Sims will happen. I do not like Sims having finals between 11 PM and 7 AM. I will move a final to 8 AM the next morning. I also have 7 extra neighborhoods in my game from Mod The Sims (Arbor Falls, Bitville, Four Corners, Garden Heights, Mesa Flats, Riverside, and Sedona).
I did not need DLC for Sims 3 because I barely played it.
And I do not think I will download anything for Sims 4 once I get the rest of the packs I do now own and finally the Kits I want. I hardly play this game as well. I am still learning and playing features of Sims 2 I had not discovered when this game was released 17 years ago (I have the Ultimate Collection in my Origin account right now).