Flex-duct as a Trunk?
There may be some opinion on top of what I believe to be true.
There are some large flex duct sizes but the ribs are horribly inefficient.
 Metal trunk is also better suited when going vertical it is easier to secure.
 Frame work is square to box-in a round trunk the box would be much larger. 
I have seen DIY main trunk in a home we were replacing the “mess” that really did not work well , added adjustable baffle dampers and the system worked where prior the furthest rooms had no flow graduated size ducting fixed the mess.
So sure it can be done but the trunks will take up more room and not work as well.