Screw-Shafts: First off, your link between the tilter and the tray has more friction than it should. I see you have a good screw shaft on your arm which my team learned from this thread. Your tray linkage should have better screw-shafts. Using a screw as a shaft only works well if you tightly secure it to one linkage/bar and then loosely secure it to the other bar. That way there is no slop, but there is also limited friction. Here is how my students adapted the shaft-screw:
nut retainer, c channel, keps, teflon washer, other bar, bearing on the other bar, another teflon washer, thin nylock tightened and then loosened to the desired effect.
You can do this where the tray linkage attaches with the tray too.
Chassis: Your chassis seems to have no good trans chassis support. I would always recommend making sure a single c or L channel runs across the entire chassis and makes direct contact with every chassis rail. The reason for this is rigidity. Your chassis is going to flex a lot without it. You can just have the chain run underneath it and rub up against the structure lightly. If you are worried about friction, give it a layer of polycarb to slide along.
What issues have you had so that we can suggest resources? Your build is quite advanced, but definitely has some opportunities.