How to excavate hole when only 30% of perimeter is accessible to tractor?
Not sure why this is a problem. You drive in and dig the farthest part first, with a lot of trips back &amp; forth to bring each bucket of dirt all the way out. Then you work out a couple of feet at a time. Exactly the same thing you would do if you were shoveling by hand and putting the dirt in wheelbarrows.
Mark the entire circle at the beginning. The biggest issue I see is making sure you don't dig too much. Unlike Kindergarten drawings, you must stay inside the lines.
Really not that much different from a much more common problem - how to paint the floor of a room. Start at the farthest spot from the door and work your way backwards to the door so you don't literally paint yourself into a corner.