Plus we have a lot to learn before we start exo-farming. It's not clear how reliably we can grow crops on Mars even in a well controlled greenhouse
Which is exactly why we should go there. Were not going to find out or learn anything without going to Mars to test these things.
Mars has enough gravity that most plants should grow just fine,
Perhaps but currently that is an unproven assertion. Frankly the gravity is likely to be among the least of the challenges to growing food on Mars. When you have a small self contained garden you run the risk of any number of problems hugely disrupting the entire crop.
Yet more reasons to go there and start growing as many things as possible, so that we can test absolutely everything.