Imho, just to bring here some design pattern recall, have a single instance of the database connection object is the best practice (I'm broadly refering to the Singleton Pattern FYI). Like this you do not have to instanciate the db object and create the connection for each api call. You will save time and resource usage.

So If I understood correctly you want a few guidelines for keeping on this project. First of all, you should plan, with some sketches, the app sections that you imagine, then go on with the coding. Like this, you can actually have a roadmap and know what to build.

Hey, what exactly are your doubts? I personally voted JS because with it you can pretty much do everything from embed system to Web app.