If we’re going to trouble of terraforming Mars, we could probably do an artificial magnetosphere for the planet. It wouldn’t even have to be as strong as Earth’s, just enough to drastically slow the loss of atmosphere down to the point where it wouldn’t matter. It would be quite large, but doing any sort of mass terraforming project of Mars would be an enormous feat in general.
In any case, it’s good to hear that Mars had hospitable conditions for the first couple hundred million years, and that it then took tens to hundreds of millions of years for those to disappear. That means if life formed on Mars in the hospitable period, it’s extremely possible that it survived to form a deep biosphere down where the planet is still warm enough for liquid water and vulcanism (i.e. several kilometers down from the surface).