I take over other civs instead of expanding to new continents. It is usually because I ask myself what would those new cities give me and the answer is very little.
Depending on how many turns it takes you to research a tech you can kinda guess how much science per turn you need from the city to not slow you down and it usually takes a lot of build-up to actually make a profit. Production is usually spent to catch up on buildings. Military cap is rarely a problem for me unlike unhappiness which is a constant struggle in any domination game even outside of war where you get even more from war weariness and occupied cities. Archeological sites are super not worth it since you can get artifacts from them anyway and the 7% decrease in tourism is really bad if you are going for a culture victory. So it is really only about the gpt and luxuries / resources.
If you need strategics (mostly coal, oil and iron are quite important and needed in huge quantities for large empires) it might be worth to place a new city if there is some really good spot where you can get about 10ish copies of it, but other then that taking over other players is much better. It usually gives you reasonably developed cities and there is a ton of stuff giving bonuses upon city conquest.
I have also placed a new city to grab a good monopoly (for example one that lets you found one of the more powerful corporations - culture, science, extra great people points), have more production towards naval units and at strategic spots to prepare for an invasion, but I don't think I ever build a bunch of mid-game expansions or even used more than one pioneer in a game.
Of course all of the above applies for authority/progress playthroughs where you often go to war, if you play a tradition game you basically never want to go above 4-5 cities since your main strengths (great people, your awesome capital with crazy tourism output) do not scale with the number of cities you have.