Hi I'm new in freelancer, first of all, thanks for the opportunity.
Actually I work with Graphql apis, but I have a great experience with Rest APIS, including, creating an app from scratch on my first job, from database, passing on rest api, to final mobile app with React Native
What I though for this project is, use typescript and jest for testing to increasing the code reliability , if it don't represents any problem to you.
I've increased the price of the project because initialy is only backend, and i can develop it frontend to if you want, using React and Material ui, creating an interface on Figma first and adjusting it if you wants.
I put 1 month to develop to prevent some unexpected changes or problems, i think it can be delivered before the prevision.
Good luck on your search of a freelancer, hope I can help you!