Wow! a surplus Ash Dome?
It sounds like your's and mine will be almost the same.
If I were to do mine again i would do a couple things differently.
Excavate down below the frost line and make a flat compacted surfaced (undesturbed soil would be best) Make the excavation big enough to work with clearance all around.
Make square forums and pour a huge block of solid concrete, about 6' square and 3' deep. Put rebar in it and sticking out the top to tie down your column. Make sure the top of the big base is 6-8" below ground level. Build a column of cement blocks about 4' sqaure on top of the big block. Go up as high as you want. Grout the blocks solid. Leave the center hollow. Use rebar in the blocks. Use plywood down from the top of the blocks about 6" and fill the top with cement and rebar tieing it to the walls of the column. Get some Stainless steel "L" bolts in a large size (5/8" or 3/4") and arrange them in a bolt pattern using a sheet of plywood with the "L" bolts bolted through it. When the cement is hard take off the plywood and replace it with a "weld to" plate (I plan to use 1" thick steel plate).
Get some sheet foam or spray foam and cover "the big block" with it thoroughly. Wrap the column with foam (I use blue 6" wide foam from Home Depot that goes under the bottom plates of house/garage walls). Wrap the foam around and round the column so it's about an inch thick. If you do it right you will be able to skreed using the top of the blue foam and the edge of the foundation wall forms
Next pour your foundation slab/walls/footings that the walls will sit on. Round or not.
All the foam will isolate the column/big block/scope from the walls/floor/dome.
Put some 2" gray (pvc) conduits in the forms so you can bring in power and communication wires
When the concrete is hard, grab the end of the blue foam that is wrapped around the column and pull it out. It will leave a gap between the column and the floor.
Sheet with corrugated steel