Let me suggest two things, but I don't know if they will solve the problem.
The first is that once you set the rubric you are stuck with it until you publicly replace it. You may be right that there was collaboration but your "feelings" can't be acted on without evidence. I think you understand that. If it only mentions quality and completeness then it is that which you must depend on. But, you could also require explanations of things as part of any assignment. In Java, we have JavaDocs for that where we describe the intent of any class and of important methods. They are harder to grade, but also harder to copy in any real way, since they use natural language.
A short exposition, say a page, of what an assignment has accomplished and how could be submitted along with each assignment. Ideally it could include the "why" of major decisions, such as algorithm choice. Students would be required to explain their work, not just provide it.
The second thing is to spell out explicitly (for the course, and perhaps for individual assignments) what sorts of collaboration are permitted and which are not. But to make this effective (hmmm, more effective) you need to get them to actually affirm that they will respect it in all details, both the letter and the intent. It is one thing to say to students "You must do this." and quite a different thing if the student says to you "I agree that I will do this."
But in any conversation (oral or written) about an honor code, you need to actually explain why the rules are in place. After all, in the real world, collaboration is expected and the quality of the product is paramount. In an educational environment, it is the learning that is paramount, not the product. Students don't always understand that you didn't ask them to do something because you want the result. After all, you could have produced it yourself much more easily. Emphasize that any restrictions are to help ensure their skill development.
And, of course, if you forbid them from communicating with one another they you almost certainly need to provide ways for them to communicate with you. My technique for this was a private mailing list that all staff and students were subscribed to. Students could ask questions (but not post solutions/code) to the list. The questions would get answered, perhaps by another student - in public. Everyone saw every question and every answer. This has the additional educational benefit that when one student asks a question it is likely that others do also, but hesitated to ask for whatever reason. No one needed to feel "stuck" and then tempted to break the rules.
Like many things, students need to be taught how to behave.
Note, of course, that the above doesn't address "originality" explicitly. If you want to encourage that, you may want to find a way to get that idea into your rubric. But you would need to be careful to define what you mean. Variable names taken from Pokemon probably isn't helpful. One way to make this work is to offer a few points in the rubric for "extensions" to the assignment. They would have to be explained in natural language at least, and, perhaps programmed. But a few points for suggesting some "novel" extension to the assignment might work and be hard to fake if arrived at collaboratively.
