Thanks for the feedback!

Using two COM pins still makes sense to me; fitting six wires into a single screw terminal could be a real pain.
The outline fits the RasPi fairly snugly, so my size goal is pretty much met. I’m not too worried about the space taken up by that extra COM terminal.

Assuming a good internet connection, rainfall/humidity/temperature information can come from a local weather station. An analog channel could be used to detect open circuits on the outputs (a single ADC channel should be enough, monitoring a sense resistor on the COM terminals).

A few more ideas for additional features:
1) Interface circuit for an electronic flow sensor. Many deliver one pulse per volume unit of water – if I can’t find a counter with I2C or SPI, an ATtiny would do the job.
2) Indicator LEDs for each channel. Space is especially tight around there, so it might not be very practical.