How to order CKEditor plugins
Indeed, all you can do is specify the other CKEditor plugins your CKEditor plugin depends on. It is guaranteed that those are loaded first. But beyond that, there are no guarantees. Nor should there be.

So, just specify the CKEditor plugins your plugin depends on in getDependencies(). This ensures Drupal informs CKEditor about where those plugins live.

Then, it is up to the CKEditor plugin itself (i.e. the JavaScript code) to specify the necessary dependencies, CKEditor then ensures the depended plugins are loaded first.