I have read through a couple of pages of the assembly so far.
A friend put me onto your site over the weekend.
Looking at the design and the bearing mounts if they are perpendicular at 90 degrees(Some look to possibly be 60 degree but I have to check again) could you not use square tubing for conduit? I think it would be more ridged along the run as long as it it not twisted for larger builds.
Pound for pound the round tube wins for rigidity but I have read people filling the conduit to reinforce it to prevent sag, which defeats the purpose of light weight.
This comes up a lot. Square takes more hardware, 4 of everything instead of 3 plus tensioners, and as you said less rigid. Filling it should not do much/if anything for the initial sag flex maybe for large overall flex but the flex we deal with hits before the filling would. The easiest way to get more rigidity is just move up a size.
All of that goes on the assumption you need more rigidity, I can easily make it fit larger conduit but have never found the need.