It seems to me that, once you have a trim tab arrangement set up on your rudder you could also use a horizontal axis vane to drive it, either via crank or using a small quadrant.
This is how James Wharram's simple design works on his catamarans which have a similar rudder arrangement to yours.
One advantage is that the vane can be remotely mounted if you have a real estate shortage on the stern.
Of course you could do this using a vertical axis vane as well, but I believe horizontal vanes are favoured for more power and accuracy.
Wharram's website has the design available for simple construction.
You can see it in operation
HERE
https://youtu.be/HwEejk7bhCw?si=Ij-qNdIn2fUwmFLO