As much as you probably aren't going to like the answer, your best bet is probably going to be swapping valves for solenoids, which gets even worse if you need to maintain proportional control. A lot more heavy equipment is going to "fly by wire" using these, so the technology is available & proven (and not cheap).
Another possible option would be using linear actuators instead of servos but I'm betting they are going to be too slow to be practical for whatever you're trying to do.
So - sticking with servos; the main issue to deal with is Force vs Throw aka the basic lever principle. The longer the lever the smaller the required force and the greater the throw.
On the valve side longer is good because it means less force & bad because it means more throw
On the servo side shorter is good because it's higher force but bad because smaller throw.
I found a large scale servo rated at 33kg-cm and 60deg Range of motion (This is just a "for instance" to get us real world numbers, I'm sure there are 1/4 scale and larger servos with better capabilities).
Turnigy TGY-S8166M Servo Specifications and Reviews
60 Deg RoM makes finding the throw vs lever arm pretty easy since it's an equilateral triangle (60 + 60 + 60 = 180 so all angles are equal thus all legs are equal length)
The total length of throw will be equal to the length of the lever arm, however since you're pushing / pulling in both directions (30 Deg each way from center) then you get half the throw in each direction.
So - putting real world numbers back in:
33kg-cm means you can generate 33kg worth of force at the end of a 1 cm long lever.
A 1 cm long lever (by properties of an equilateral triangle) gives you 1 cm total throw or .5cm in either direction from center so you can push or pull 33kg for .5cm.
Making the lever arm longer will increase the throw & reduce the force. Thus a 2cm lever arm on the servo would result in 16.5kg force for 1cm in either direction from center.
You're going to have to play around to find the right lengths for both to get the desired operation, and you may even need to engineer one or more intermediate levers to get the right balance of Force vs Throw.
I think I would start by putting a very long lever on my valve and using a spring scale and ruler to measure the required force & amount of throw for every 1 inch of length on the control lever and then work my way backward to what the servo can deliver. I would also see if I could get by without full valve operation / lever throw.
Sounds like a fun project whatever it is. Best of luck.
