Terraforming Mars is one of the best designed modern board games, in my opinion. On your first introduction, it can be overwhelming, and there’s likely a bit too much to learn for some. But I think the goals and points awarded are very well suited to the theme, and the overall framework and mechanics are not that complex. They just designed them solidly, and layered on good ways to use those mechanics. Our neighbor friends managed to learn the game solidly on their first play, and so did one of my nephews. Heck I think he had the game down by his second turn! My sister wasn’t sure about it on her first play, but she won her second game solidly! (Much to my dismay ;-P)
The two expansions our friend gifted us add zero complexity “in the moment” - that is while taking each turn. The maps just add some variety to the tile placement, and mix up some awards/milestones you can work towards. The Prelude expansion again adds variety, with some new starting corporations and projects, and a bonus step in the beginning that kick starts your “corporation”, which actually often helps newer players pick a direction to head off on. (There's a third expansion that adds a Venus board, which does add a little more variety and some new goals, but the Venus/cloud mechanics are really just another skin on the existing mechanics! I know much less about the Colonies expansion.)
I’m sure the game isn’t for everyone, but a lot of people have at least a mild fascination with something like life on Mars, and the “science” that the project cards includes isn’t something you have to really be interested in to enjoy the game play. I actually sleeved the cards for the first time ever, and ordered upgraded player mats, as you do have to “track” a few things, and the default player mats are just flat cardboard that your tracking cubes can slide around on. These upgrades hold the cubes where you place them, and also make it easier to count above 20 for people like my sister and spouse that find ways for their corporations to have really high incomes :-D
Anyway, I recommend the game, but just know there’s an initially steep learning curve, but once you get past it, everything else follows pretty readily.