If we ignore Physics then we could argue that Mars being round does not mean it is not flat it could be a circle but not spherical provided it always presented itself to us face on.
Some of the key evidence that the earth is basically ball shaped, actually it bulges around the equator but close enough for the sake of argument is the fact that when the earth casts a shadow on the moon it always looks like part of a circle. This would only be possible if the earth were flat but circular and always presented itself face on to the moon.
A second argument is that the tallest feature on the earth should be visible from everywhere, with suitable magnification, as it can not be over the horizon because there isn’t one.
Finally there are images from high earth orbit or space which show the beauty of the rock we live on. If I am expected to believe that this a a conspiracy and all the images are faked then why should the US, UK, ESA, Russia amongst others all want to peddle the same conspiracy?
I’m not suggesting here that anybody believes the claims of the flat earth society just pointing out the obvious flaws in the argument.
That said I trust both the US and Russia when they assure me that on Christmas eve they will be tracking Santa to ensure his delivery schedule will not be impeded.