I am not sure about the threshing part, but I know my Great Grandfather would not hire a hired man to hand scythe his crops who could do less then 3 acres per day, while a really good worker was one who could do 5 acres per day.
I have little interest in this myself, but
should you be inclined, I would suggest you take a look at YouTube and do a search for "Homemade wheat threshers" or something along those lines. People have been raising wheat for 9000 years, about 8800 years before McCormick came out with his thresher, so it is completely possible. I would think using a scythe and harvesting would be the easy and quick part, while the threshing would be the more difficult part.
I always thought a mechanical way to do this on a small scale production would be to own (or rent) a walk behind sickle bar mower, build a home made thresher, then get a small grinding mill.
It may be interesting to note that my Great Grandfather (many times removed, not to mention a Maine family back in the day) started General Mills in 1866 and people stated that his innovation of building steel milling rollers would never work (they used stone mills at the time). When he proved them wrong, they said it was so productive that he could never get
enough wheat to the mill, which has been debunked ever since then. I think small farm grain production and micro-grain production is one VERY under-developed area that Permiculture could thrive in and make
profit from. Who would not want something that went from soil to a warm loaf of bread?