Please don’t get me wrong but you are asking others to do the brainwork for you in advance.
I suggest now that you are able to define your requirements and you know which devices you would like to use, you should develop the basic framework from initial flow ideas and ask specific questions about the problems where you are stuck.
Break the requirement down into individual problems:
which kind of rain detection?
how to accumulate open aqara sensors? → Here I can give you a solution based on homeyscript.
sending them to Google home as an audio announcement
sending them as a phone notification
For each of these problems you will already find a solution in this forum.