A moving barge with a known flat surface of a known hardness and stability is a whole different category of difficult than doing the same thing on naturally occurring terrain with unknown voids, hardness, roughness and consistency.

Is a landing barge not "purposefully engineered to be flat"?

So what you're saying is we need to blast some barges into space and land them on the moon so we have a flat surface for starship to land on?

They also have the atmosphere, with drag that will make all velocities trend towards zero. Don’t have that on the moon, gotta do it all with fuel. More than half of the energy she’s by the returning falcon is aerobraking.

Yes, the moon has substantially less gravity but it’s also exponentially harder to get the fuel there.

"the atmosphere, with drag that will make all velocities trend towards zero"

Did you forget about gravity? Terminal velocity in free-fall is not zero!

Trend towards. The rocket is descending far faster than low altitude terminal velocity.