I'm not aware of any Ackermann effect on boats – after all, we're dealing with fluids not contact with solid surfaces like roads…. (I do, however, run some anti-Ackermann on my Le Mans-style sportscar but that's another story !).
For the rudder bushings I would use Sif bronze as it's self-lubricating, or, a good quality nylon bush (just a personal preference). The bronze should turn nicely on the lathe…
Geometry-wise, I'd go for a parallel linkage, about 30 deg's deflection either side of mid-ships. Fit control horns having a row of holes so you can fine-tune the throw. I think most people drive one rudder from the steering servo and 'slave' the other off it via an adjustable linkage rod. Use a little Vaseline (or, similar) top and bottom of the rudder posts to help with water-intrusion and lubrication.
Let the experts advise you on paint-cracking /hull preparation as I've no real experience here. (A few close-up photos would help here I expect).
Dave