Sand method will collapse or tear...take your pick.
Not based on the drawing in the 1st post. It might not even be needed with a proper bending mandrel. Years ago we had to do some large radome cable bends. Huge coax. Had to design and build a bender that had an inside spool piece and an outside sort of channel both to the right side. Then all you had to do was run a bearing at the right radius off of the capstan center to the outside of the channel piece and run it around. It held the entire diameter tightly and so kept the buckling from starting. We could get much tighter bends without collapse that way.
Another method patented in the heating appliance industry uses a sort of finger or spoon like shape in the bending die inside where the pipe is fed into the machine (a friend of mine was the inventor). This guide touches the outside of the bend from the inside to keep it from collapsing and guides it as it is stretched by the bending force. Hard to explain in words but you can get perfect tight 180 bends in thin wall with these machines. The tubes were used for heat exchanger tubes in furnaces so the tightly packed S bends give lots of path and surface area. These machines were manual at the time but work just like CNC benders today, sorta like a spring forming machine today only with tubing.
Just thought of the other method probably not appropriate for tubing of this size... But using a spring of about the correct inside diameter you insert the tube inside the spring and use a mandrel to bend both the spring and the tube inside. The spring holds the tube round while you bend. Then you have to get the spring back off afterwards. Can be done with the spring inside as well. I would worry that you would collapse the tubing inside or outside the tube and have a hard time removing but maybe a little planning and it could work. Normally this sort of stuff is used to hand form smaller tubes to largeish radii. Maybe just food for thought that can be bastardized in some way.
And the last and most common DIY method is to just buy a kit of exhaust parts from any of several suppliers. A few hundred bucks and you have enough 45 with straight on both ends. 90's with the same, 180 bends, plus straight, etc... to make any exhaust shape. You have to simplify your centerline path on your drawing to use the radius you get in the kit and you start cutting and tack welding until the ends and middle meet the specification and clear all the obstacles. Ebay full of kits like that in various materials and diameters for making headers and full exhaust.
Of course these are all cold forming methods. Get out the torch and another world opens up.