I highly recommend the Elixir for Programmers course by PragDave. It gives you a great framework for building a well designed, modular application in the vein of Phoenix is Nor Your Application.
For books I would recommend Programming Phoenix
Many other resources can be found in this thread.
Start building your app!
This is definitely a good fit for Phoenix. I don’t think you will need much concurrency or distributed systems for this app until you get to a serious scale (that the vast majority of projects never make it to).
Similar to the last question I don’t think you’ll need to worry very much about OTP until you start having a need to scale your app.
Overall I think you are on the right path, especially if your goal is to learn as well as to build this app. The stack that you mentioned looks good as well.