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.