Ian,
In the last 35 years I have tried most of these systems and they all suffer from slop. At model sizes we are always going to have to compromise to get a geometry that will work.
Torque tubes can be made to work to a degree, but need to be engineered extremely well.
You cannot beat a direct short linkage. I would apply the KISS principle personally.
What is the model of BTW?
Regards,