I have many experinence deploy web map application using google map api. mapbox, arcgis server. As your description, I think You'd like to build nackend system can make route search and you can modify, so google map API cant request route when road close. I think the soluttion is make a search system, there are 2 options:
1. use Pgrouting. (opensource, free)
2. use OSRM. (opensource, free)
It can apply in your project. I'd like to discuss more with you. Feel free to contact with me. And we will deal exactly milestone and price.