During the summer I have worked on the first part of a watering and climate system for the greenhouse.
The basic criterias are
Fully integrated with Home Assistant
Totally independent of Home Assistant, wifi/network or internet
Reasonably robust and redundant
The plan is to use several esphome nodes communicating between each other through hardwared RS485 for the core functions, and wifi to Home Assistant for info and notifications.
So far I have done the first part, one esphome controlling the pump and valves for the rainwater tank.
The tank is a 1000 lirer IBC-tank with a hydrophone pump.
I have mount a ultrasonic distance sensor in the lid and a “low-low” capacitive sensor in the bottom of the tank that cut the power to the punp to prevent dry running.
The “pump-esphome” controls four magnetic valves for three watering zones. One for inside the greenhouse, two for outside and the last valve for dumping water if the level is to high in the tank.
To open a valve for watering a command must be sent through the RS485 interface or from HA.
The valve is only open for 5s and to keep it open longer the command must be resent within eery 0,5 - 5s, Outside that range the valve will be closed after 5s.
I need some adjustments for the tubes that fills the tank with rain water as they sometimes cause interference to the ultrasonic level sensor.
The next step is to build a second esphome unit inside the greenhouse that will request water from the tank through RS485 and distribute it to different plants by a four way valve.
A third esphome is planned to control the climate by controlling a heater and a fresh air fan.
We have a Underengineered Watering system in place and it got quite some things in common with yours actually.
Our tank keeps around 12.000 liters of water and is open. A D1 mini with a ultrasonic sensor and a tilt switch to monitor the water level and signaling when full to the pump (another esphome node).
The nodes controlling the DC valves we have look something like this
And have only the bare minimum of components needed to do the job. Essentially D1 Mini, DC Step Down Converter for the esp, 4 Relay board and some terminals In total with case less than $10
This was the first unit I build years ago and still goes strong today It also has a flow meter connected
In total with case less than $10 
It also has a flow meter connected 
