Fun!
I let it run for a long time after the last battle to see what happens; at some point there is a lot more gold than grain; I upgraded villager speed to the point where the game didn't have a name for the upgrade anymore, but I had loads of lower level grain upgrades still left to go.
-> I think it might be a good idea to have a button to stop grain trading temporarily.
I won the first attempt because I just happened to run into arrow range upgrades naturally, so I don't think it's too hard ... too random maybe, but that's kind of what this style of game just is.