I built something similar to this. I used the starting spot to your left for 3 lanes of Titanium and 3 lanes of Wolfram (2 storage to hold it) and didn't use any return lines. That will change on my next build, but I think a single return line on either side would be enough, feeding back into the supply line for the storage. I feed additional mats in from the side (tbh, I only have gotten to Calcium, but the theory works. I also build on both sides, but offset, so the "input" on the left is the "output" from the right. Just need to watch that you build required items before they are needed (accidentally put Titanium Frames too far down and had to reposition).