The shader cache is something tricky, gave me problems as well. In summary you need to put all materials and shaders visible in the camera/viewport of the player the first frames. It must be visible and you must have the materials saved, unique materials doesn't work well with cache. I rotated and scale things in the common shader so it was visible in the low corner of the camera.
For the milling machine I expected that it automatically open up. like the animation that it does but in backwards. But yeah it need a sound too.
For the arms, that's interesting! something that we don't know because people are all different shapes haha. Also I played in stand mode, if it's usefull for you.
The difficulty is something common, and I didn't expect it to be right in 7 days, I think nobody would expect that.
I did a job ''roguelike'' simulator myself Gem Frenzy, adding to the formula money and upgrades. So that's very very hard to do right, it need a lot of iterations.
My humble advice with this is: You as a creator know everything of the game and had tested so many times that you are a pro in your own game, the players will not know everything and all the mechanics (or almost all) will be new for them, so what you feel is easy, for them would be just quite nice, and what you feel is quite nice, for them would be hard, and with iteration comes experience and a little more well intuition of what's right.
But yeah, nobody can make it right in 7 days or less