Re: " Getting a sufficiently thick atmosphere would require"
And you also need to keep it there.... which would require a magnetic field as well to shield it from Sun energetic particles. Which also are not nice to life forms
An Earth-like atmosphere offers radiation shielding similar to about 10 meters of water, which will stop any electrons, protons, or x-rays from the Sun. With sufficient oxygen, it will also screen UV radiation, too.
That does leave the problem of solar wind erosion of the atmosphere, but Mars should be able to hold an Earth-like atmosphere for about 100 million years. That moves the issue of atmospheric erosion firmly into the category of "let the grand kids deal with it." ;)
