Roof mount a schedule 40 PVC pipe, looking for angled/adjustable roof flange
I'm having a bit of trouble figuring out the terms. It seems that when you write &quot;roof mount flange .. meant for exhaust ductwork&quot; you might be referring to a roof flashing like this: (image from oatey.com)

Those flashings aren't intended to support a pipe at all; they merely keep water out of the roof. I don't think they'd be of any use for mounting your pipe.
When you write of &quot;supports from an old satellite dish&quot; I think of an antenna mount like this one made by Laird: (image from streakwave.com)

I can see the foot of that mount working well to anchor the bottom of your mast. I use those alone for mounting small (14 inch diameter) antennas, but for your taller mast, a brace like this one used for satellite dish mounts might be useful: (image from summitsource.com)