How to execute commands on a 3rd party app using adb commands or something similar

I have a use case which requires an app, not developed by me, to be controlled. I have adb access to the android device. These are the steps that are required to be performed:

1. Input text in two fields
2. Press Enter
3. Select checkboxes
4. Enter text into another text field
5. Enter

I know I can use adb input keyevent <event-code> and adb input touchpad tap <x> <y>, but these are brutish methods that won't work every time. I have looked into appium, but it seems to be limited to mobile web applications. Other frameworks like Espresso work, if you've built the app yourself.

I could not find any good answer to this problem. Any help is highly appreciated!