Update: I hit tier 3 today. What I decided to do was just farm for copper (and tin, which pops up when you're mining for copper) and for awhile set 2 furnaces constantly all working on bronze bars, always topping them off each time I picked up any, and turned them all into bronze pipes. Doing the math between bronze pipes and plates, it equated out to a value of 26g per either way, so it made more sense to go for pipes since they only required 2 bars per pipe to craft versus 3 for the plates. I kept 1 of my furnaces open for crafting guild commissions, usually glass or stone bricks since I wanted to save my copper for the more profitable bronze pipes. Selling these to the guild every time I popped in to grab or turn in a commission, I racked up the money pretty quickly.
If you are able to find the small engines in the mines (I can't seem to so far, and I've been looking nonstop), then you could also build the dee dee vehicles. Their value is 2800g (seen on youtube where someone completed one). It doesn't appear that they are usable in game yet, so might as well sell them. I found the most time-consuming part of trying to make mine (only missing the dang engine ugh) was getting enough of the fibre plant to make 20 fibre cloth for the top, but you can forage a few of those each day usually.