Have you considered testing a triangle structure as top ring? I know you’ve tested a triangle torsion transfer tube. No need to replace the tube, but just the ring with the propellers. Needs new 3D printed parts though. (Can do quickly in Fusion 360, if Rhino is making trouble) Bridle the triangle corners to three or so that when in operation all tethers are under tension. Not equal tension though due to trigonometry, except if you make a load distribution system.