Tourism is something you have to intentionally build up. You will initially only have a few applicants, and if you provide them interesting sightseeing tours and a comfortable living situation, they will generate more applicants upon leaving.
Concerning the expansions:
- Space race mostly fleshes out the different sponsors. There are some ways to interact with rivals, but for me the main draw is stuff like the Church of the last ark's temple spire or the Japanse wasp drones. A bit of gameplay but mostly flavor
- Green planet defines an end game goal after the "survival" is fully secured, which vanilla S:M is somewhat lacking. There are also some interesting strategies with early open farms, but it's mostly about the end game
- Below and Beyond is about making the mid-game gameplay more varied. S:M can have some slow phases, but with B&B you can always switch to another "plane" (underground, asteroids) and advance your goals there. It was somewhat unfinished and bugged upon initial release, which you might notice in the form of some bad steam reviews, but in my eyes it is now a fully fleshed out expansion that integrates nicely into the main game.