I’m a software developer just starting with open sprinkler systems, so take what i say with a grain of salt. In your scenario I would go with only the pi-powered open sprinkler controlling the valves and also connecting the sensor to the pi. A linux service can easily be run on the pi to do whatever work you want it to:
– wait until full
– if it rains or it will rain in X hours stop for y hours (optional)
– get last used valve id
– increment the valve id
– start watering
– wait until Z minutes have passed or sensor empty
– start all over again