On the atmosphere point that's one thing that isn't as much of a problem. Plants can breath in Mars atmosphere and pressure.
Sure the pressure is 0.6% comparing Mars/Earth but the CO2% is 0.04% when comparing Earth/Mars.
In other words the partial pressure of CO2 on Mars is actually higher than on Earth. Which is great for CO2 breathing plants. If the temperature was raised a plant would have more CO2 available even with the much lower pressure than on Earth.
Indeed experiments with 95% CO2 (matching Mars) and 6millibars pressure (matching Mars) and with martian soil showed that Potato's grow fine. https://cipotato.org/blog/indicators-show-potatoes-can-grow-...
The only adjustment was temperature!