If you sign in to the Projucer with a paid account then you’ll be able to disable the splash screen and analytics collection. You can also manually disable them using precompiler macros in your app’s AppConfig.h without violating the license agreement.
Just as a reminder: you’re free to modify JUCE however you wish while developing. It may be best to start paying for JUCE at the point at which you release your first product. See the license agreement - https://juce.com/juce-5-license.