Regarding rocket propellant

Methane is more dense than hydrogen

Methane makes more thrust than hydrogen

Hydrogen rockets need to have huge fuel tanks, hydrogen rockets have low thrust - the space shuttle needed boosters to lift off

Everything in space is a trade off, kerosene is better in all those ways than methane, but you can make methane on Mars. The global warming potential isn’t much as there are so few rocket launches, and hydrogen fuelled rockets use fossil hydrogen (made from fossil methane (aka natural gas)) because it is cheaper than alternatives