you might consider installing a Johnson Controls A350 and a 24v/0-10 actuator on the existing dampers. Strap the sensor that comes with the controller on the liquid line and set the controller to your desired liquid temp.
I was thinking about how to do something like this, but by using liquid line pressure. I haven't been able to find a device that would output 0-10vdc or 4-20ma based on a pressure range suitable for this application.
Have you ever done this using a JC A350 and LL Temp? Did it work well? How fast is the response time?
No I have not used an A350 for thit before but if you insulate the sensor good it should work fine. The 350 has some tuning adjustability and you have a TXV application so I would prefer to have as constant a liquid temp as possible.
I would probably pay the extra for a spring return actuator and leave the Penn control on the fan as well.
I'm good at making things cold...You can ask my first two wives!