Im really confused as to why so many people are excitedly listing what features they'd like this new sprinkler controller to have and then the OP is talking about making a brand new one from scratch and im honestly baffled here because.........
All of these features are included in the default
Esphome Sprinkler integration.
So are all of these....
All of these are included by default except a battery monitoring configuration but, that is covered by itself already in the official Esphome documentation and one would just need to add it into their config and set it up which is very easy.
Same story for this wish list too. Its already all built-in and called Set Divider
I worked in landscaping/hardscaping for 15 years and installed 100's or brand new lawns and hundreds of overseeding jobs and im quite sure that this isnt even a thing that people do or is offered by the professionals responsible for installing the turf/seed. If for some reason there was a problematic head/zone thats making a mess and blasting holes or causing damage then the right thing to do is replace or readjust that head/zone and also, most systems arent even capable of adjusting a zones pressure like that for a "gentle mode"
This isnt a default functionality but, you can very easily import sensor data from elsewhere like in HA, MQTT, etc. You can easily create whatever entities/sensors you'd like such as rainfall sensors, sensors that track and calculate previous day/s rainfall totals and forecasted totals and then use that data to more accurately determine if and how much irrigation is needed based on say, last 48hr rainfall + today's forecasted rainfall with the fact that today was gloomy/overcast all day which would significantly make a difference in choosing to run a normal cycle or a much reduced cycle. Hopefully that made sense...
This also is not an included by default functionality but, just like the avove scenario and example i gave, it could very easily be something that could be manually added.
Heres someone else's completely DIY built sprinkler where they used the Esphome Sprinkler Integration thats already included by default.
You can also find tons of historical examples from many other people over the years and you can find lots of cool systems, designs, ideas, etc and the best part about this is no one has to reinvent the wheel and build another one from scratch when one already exists!