The decision to postpone the test or not comes down to a personal question? Will you be ready? The answer to this question depends on how confident you are about your current study and whether you believe on the planned date, you will be able to hit your target. No one here on the forum can answer that question for you.

But we can help provide some insight though. What your was baseline score when you first started? How long have you been stuck in the mid 500s? What techniques are you using to overcome your plateau?

Are you very methodically going over your mistakes to develop lessons that you can learn from? See my blog for an example. If you are not then you need to do so because it is, in my opinion, the best way to overcome glass ceilings in your studies.