I live near Pacific City so I can answer questions for you.
My suggestion would be:
Plan for September, May can be too rainy.
Think about a vacation rental home to rent and use that as a home base. You can make day trips within the area you talked about. There are vacation homes up and down the coast. They can be cheaper in the long run.
There are many events taking place in September at the Coast that you can partake in or watch and enjoy.
All the Counties and Cities have tourist board so go online and get as much information as you can.
Paul