I agree, this is amazing work. You’ve accomplished more by yourself on a hobbyist’s budget than some agricultural machinery makers I happen to know with half a million Euros to spend. Looking forward to following your saga! 
BTW: I’m usually working with robots that are much, much smaller. And I’ve inadvertently run them into, over or under things many times. The thought of having a tractor run autonomously without any collision avoidance scares the shit out of me. Since you’re working on an open field, implementing this would be trivial if you just had the right sensor. I see you’re looking into Radar or lidar sensors; that would be ideal. If money wasn’t an issue, I’d probably go with the SICK LD-MRS. Despite being a Lidar, it can scan through dust, rain and snow. Or perhaps a Velodyne Puck, which would give you 360° vision; but that one’s also about $8k I think. So perhaps Radar is the better option.