Howdy. Happened by this site a week ago while I was dreaming about doing some blacksmithing. I'm not a blacksmith or any sort and have only started to do some metalwork with an oxy-acetylene torch set (eventually want to braze a lug bike frame together.. but I digress)
Anyhoo.. saw your post and wanted to suggest you take a look at "modular spiral staircases" (google it up)
Spiral staircase and modular staircase kit products (has installation videos also)
They are simple construction where each tread is separate and has a heavy tube attached. This is your main support for the treads. The tube usually slides over a central pole. There is usually a small connection at the outer circumference between treads to hold them in place. This design keeps the outer circumference very light as there is is not a heavy helical beam there.
It appears some rely on the railing for support also (at least on that linked site)
I saw one being installed many years ago on TV (I'm a This Old House type person)
Anyhoo.. good luck in your venture.