Backend for planner/scheduler app

Hello everyone!
I want to make a lesson plan app for ios/ipadOS for my engineering thesis. The main goal is to have ipad version with admin panel for creating plans and ios one for users to look at the plan with some filters. I did similar webapp for my group projects with java backend and postgres(about 5-6 tables).
My question is what is the best solution to make backend? Is cloudkit totally fine for that purpose of storing and sharing data between admin and users or is there better/quicker solution? And will i be able to show demo without submitting apps to appstore?

There is no one tool to solve all problems. The selection for your backend will mostly depend on two things:

  1. What solves the problem best?
  2. What you know now or are willing to learn to make this happen?

Since you mention quicker I assume you are time constrained. I would personally then recommend adding an API to your Java backend with PostgreSQL and consume it with a iOS application. You already know how to work half the solution, arguably the most time-consuming one.