I mean...
Culture: You have to draw more tourists to your country than any country has domestic tourists.
Science: You have to launch a spaceship that travels to an exoplanet.
It's quite similar.
You're probably thinking something along the lines of: "You must generate tourism from improvements, wonders, works generated by Great People and special units, which provides tourists from civilizations you have met. Meanwhile, civs generate domestic tourism from their culture generation. Your tourists from foreign civilizations must exceed the domestic tourism of all civilizations."
But I'd argue that's equivalent to: "You must scientifically advance to the end of the tech tree. Then, you must build Spaceports. You must first launch satellites into orbit, then continue with a moonlanding, and then start preparing for an exoplanet expedition. Each next step requires more technological progress. After launching the expedition, you can speed it up with special projects."
I don't think the culture victory is inherently more complex once you understand it, and I think I know a lot about how it works, but I could not tell you how they calculate "domestic" vs "foreign" tourists other than on the culture screen. I'll go from 58/114 one turn to 62/115 the next turn, then 63/118 the next, etc...
The space victory is pretty clear "you have X pieces finished, and your journey is 12/50 turns complete, travelling at 3 light-years per turn". I an go to that screen and know exactly where I am, exactly how long it will take, etc...
The culture values jump around, you have all these individual modifiers. For some reason one civ I have +22 foreign tourists in, another civ I'm at +12.
I know people often hate some of the board-gamey nature at times, but for something like the culture victory, it might not be the worst to turn it to more of a "victory-point" style system, maybe. Make it a clear system - for sake of argument, whoever has the most great works has 1 VP, the most wonders has 1 VP, the most seaside resorts 1 VP, etc... With some bigger bonuses, like total counters for tourism output that give you another VP every 1000 tourism that you accumulate, and have it as a race as first to 50 VP or something like that. Maybe that system is dumb or whatever, but at least it would be clear in how you progress, who is winning, how far away you are from the goal, etc... Similar in ways to the diplomatic victory - you accumulate points in a certain way, and if I see I'm at 16/20 points, I know exactly how close I am to winning (other than knowing that the next congress resolution I'll have the -2 points against me resolution come up, etc..). The culture victory now especially is awkward because their "prediction" counter is always jumping all over the place. Like once that counter shows up and says I have 50 turns to victory, I know I'm actually like 15-20 turns away. And when it says 5 turns, I'm either winning in 2 turns or 20 turns.