Markus Rieger used to do a really neat encoder based piston tank control board which was fully solid state- no relays. I have a feeling they're no longer available.
Engel have sold a proportional boards for years, but up until the unipro, they were always based on a potentiometer for feedback.
Something you do need to be mindful of with a piston tank system, is that the air compresses behind the piston as the tank fills. Therefore it's important to ensure sufficient volume to prevent too high a pressure building up inside the cylinder, which will load the drive motor up and place a burden on the shaft and hull seals.
Generally the tank should be sized at about 10-15% of the cylinders volume if you wish to keep pressure within a couple of psi. For some boats with a high freeboard that can be difficult to achieve, so sometimes a hybrid system employing a second system (e.g. water pump, gas, compressed air etc.), with smaller piston tank for trim is employed. Another method employed by Ron P was a valve system which vented excess pressure as the tanks filled, and then allowed fresh air in when the boat surfaced to break any vacuum that developed inside the boat.
Piston tanks are quite popular here in the UK. People use Engel tanks and Ron Perrott produced piston tanks where the threaded rod stayed fixed, with the piston running up and down- this meant you didn't have to allow room for the rod to retract.
For smaller boats, converted syringes make excellent tanks, or they can also be used as a trim tank for a hybrid system on a larger boat. The neat thing with such a small tank, is that the small size of the motor required means they can be driven using a standard servo amplifier board. With servos available for a pound or two, that's a dirt cheap way of getting a proportional piston tank.
4" (100mm) is the absoulte minimum diameter for an engel tank, they just won't fit into anything smaller, this is largely to do with the way the motor is offset from the tank itself. Most of Engels rnage are dry hull based, but their Type VII most cetainly isn't.