Your write-up seems to cover most of the points you want to do.
Put together all the pieces that you can .
Test each piece and as you go your system will take shape.
One of the things we have learned from experience is that the end result is often different than what we expect in the beginning.
We learn from our work. we adapt, make changes and in the end get the job done.
Get a rain sensor, get an Arduino, get a relay, get a water valve.
You should be able to connect all those things and make a basic system work.
Then you can add more sensors, different timing patterns and different ways of watering such as your rain delay.