Is there a reason to upgrade lumbermill past level 1? As far as I can see the only advantage is to get more work orders, you start with 7 which get filled every 3 hours so as long as I feed the machine every 24 hours I get the same amount of resources as I would if I had 14 work orders and checked it every 2 days. The only advantage I see is that you can cut down bigger trees for more lumber but I can run around shadowmoon and snatch up trees left and right.
There is a follower who increases lumber yield to 30 instead of 20 but you can use him in a level 1 lumber yard just as easily as a level 3.
So why have a level 3 lumber yard?