So the whole reason for spending years redesigning new rocket engines that use methane instead of kerosene was that mars has CO2 which could be used to make more fuel. Methane has lots of other downsides which hurt the viability of the ship.
But if the moon is the target, that goes out of the window and they could have just scaled up the existing F9 rocket design and could have been there years ago.
That not correct. Methane is very good even if you only go to the moon. And if you produce fuel on the moon its still perfectly reasonable option.
And also Raptor engines were not a redesign but a completly new design. Even if you stick to RP-1 they would have moved to a close cycle and a new engine.
The main reason for methane is reusability. There is a reason RocketLab, BlueOrigin, SpaceX and Stoke all switched methane for next generation reusible engine.
And no you can't just scale up F9. That complete nonsense. at most you could build a new rocket with the same fuels and materials. But even then, the difficult part is the reusable second stage and that is a 100% redesigned.