Hi Bojan,
I had a 12 m length of 75 x 75 x 6 mm angle iron bent by Uneek Bending in Dandenong South. The resulting ring (3.6 m dia) was too large to trailer home in one piece, so was cut in half for transport then welded on site. From memory, supply and bending cost me <$300 albeit nearly 10 years ago. Uneek Bending are specialist benders used by heavy industry in the Latrobe Valley.
Also check out Rod Stubbings website:
https://rodstubbingsobservatory.wordpress.com/dome/
I can't recall how he bent the pipe (your turntable design sounds identical to Rod's) but I'm sure that he'd be happy to advise you. There is a good picture on the home page. I think his dome is about 3.5 m dia and it turns like a dream.
Good luck, Chris