I love the game's modeling of building up infra on Mars, but one key component seems to be missing: how do the rockets get fuel for the return trip to Earth? Because the game says samples are being returned, and I assume we're not just throwing away the rockets after they land.
SpaceX is proposing to solve the problem by first electrolysing water to get hydrogen & oxygen, then using the Sabatier reaction (CO2 + 4H2 → CH4 + 2H2O) to combine the hydrogen with CO2 from the atmosphere to create methane. Combine that with liquid oxygen and you have methalox, the fuel used by the SpaceX Raptor engine.
The game already has hydrogen, oxygen and carbon dioxide as resources, and electrolysis as a process. So all that's really missing is a propellant plant to produce methane as a new resource, an atmospheric carbon dioxide extractor (could also produce nitrogen as a side product), and a liquification plant to turn methane and oxygen gas into liquid methalox.
As rough ballpark numbers, you would need 240 metric tons of liquid methane, 860 tons of oxygen and a total of around 16 gigawatt-hours (spread across the entire process) to fuel a single rocket. More details in this study.
It is true, we completely ignored the actual supply shuttles & their needed infrastructure and logistics so far. We have a building of type "landing pad" in our feature list. Constructing this building would then be a prerequisite for receiving supply shuttles in the first place. We have added your ideas around rocket fuel infrastructure to the landing pad idea making the entire feature more complete.
Thanks for your suggestion and happy settling,