You’ve made it this long without ever playing Catan? What kind of nerd are you @Sam ?
I kid.
But Catan really is a great game for beginners. I think it is a bit of a gateway drug into more serious board games. The expansions add varying amounts of complexity to the base game.
I have tried combining some of the expansions and honestly don’t think it is worth it—especially for new players. On their own each expansion has enough replayability to keep them enjoyable.
I’d say @Aero_Engineer is accurate. Most people are going to want to stay away from the 5-6 player expansions. If you do want a 5-6 player version of Catan just grab the base game extension. There’s enough replayability there.