First off, I'm not that great of a Civ 6 Player and I'm not being modest. But I keep running into the same "problem" playing in Deity mode.
I play on the biggest map with at least 20+ civilizations on Deity/Marathon. After some interesting confrontations with a few civs early in the game (after beating them) I always get into a lobsided game where I've got 5-8 cities (because I've conquered some early game) and my opponents don't. From there it's nearly impossible for them to catch up as I simply steamroll through weaker civs.
Are there some nice mods that will give the AI some added juice later in the game? I'd like to be able to take on massive opponents versus steam roll everyone.
Use the editor and give every civ extra starting settlers. Ten should do. Then settle your first city around turn 10 or so.
edit: @Socrates99 makes a good point. Pick Russia, Rome, India, Colombia, Aztecs, Korea, and Netherlands as ai civs. If they have +10 settlers each, that should be an interesting game.