Don’t overthink it. All the rain means is there will be some road spray so bring glasses and a cycling cap, it will be colder, you have to brake sooner for things and you go around the corners slower.
If can naturally adjust between a dry and wet training ride then I wouldn’t worry about rain in a race. You are going to be nervous regardless of whether it is wet or not so just go and give it your best shot, first road races are just learning experiences and becoming comfortable in that environment.