Have you considered driving this with motor?
I think this tripod is not sturdy enough, it will move as you are turning the knob.... resulting in images worse that you already have.
This thread will have to move with very little friction.
Some sort of
1 rpm motor (powered from mains) may be a quantum leap from this design.
Or
his one (requires transformer 240V -> 12V AC)
(This particular motor from ebay may be designed for 60Hz mains.. so it may be running 20% slower in AU, requiring 20% shorter arm of the barn-door tracker).