I was looking at your sketches, before going into the rendering part, I think your ellipses and perspective could use some foundation work :
Consider 2 popular methods of drawing ellipses in ID:
One is through the minor and mayor axis (the mayor axis points to the horizon and the minor axis bisects the ellipse in 2 equal parts if memory serves), and then eyeballing the ellipse on perspective, a vertical axis serves as a place to hang your ellipses, this is a very quick way of doing it, but lacks in precision and I usually use it when objects are floating in mid air, The Sketching series of books uses this a lot.
The other that I use (they are really the same if you think about it) , is starting with rectangles and a grid and extending them up down sideways etc with Scott Robertson black magic perspective tricks , whenever I need an ellipse or circle, I use the cross method from David chelsea, that is, divide a rectangle and on the diagonals mark a point roughly 2/3 rds from the center , connect this with the midlines and you have a pretty good circle or ellipse in perspective. Notice that no matter which method I use, I connect each intersection of line-ellipse point to it's brother or sister on the following ellipse, even the ones that are hidden by the object itself, keeps me honest.
These are just 2 methods, but what I've learned is that when I am guessing I loose the drawing, so I tend to rely on the second one more, which in turn generates some truly hairy underlays.
Edit: The work of Hakan Gürsu comes to mind for examples of more accurate underlays/inspiration: https://www.instagram.com/hakangursudr/
So now that we got that out of the way and I ran out of paper, the corners or elbows of tubes are tricky because our brains suck at estimating the foreshortening of a curve in perspective, so we under/over estimate, consider the tightest elbow ; if seen from above it is a section of a circle, yet most people draw an almost 90 degree angle, to correctly display this type of elbows,use the 2/3rds ellipse trick on both sides of the bending tube and project this into the middle line.
Notice btw that the line on the tight U bend you are after is first an artifact of geometry ( on the 3d model) , but is also accentuated by shading on the render.
Hope this helps you fine tune your underlays.