Two months full steam studying GMAT for > 650 to apply HBS third round

Dear martijn_de_boer,

I'm happy to respond.

First of all, as you may know, you can press the "help" tab on any page inside the Magoosh product, and this automatically will generate a "help" ticket. Alternately, you could email help@magoosh.com. Our student help team usually responds inside 24 hours. That's a much faster way to get information. If you post here, it may take up to a week for me to see it.

My friend, if you don't mind, I am going to correct some grammar mistakes in what you wrote, because I assume you are interested in learning:
1) they invited people like me, who have successfully took the online HBX Business CORe
Even the pronoun "they" is suspect, because you mentioned a department, not the people in the department. This is fine in colloquial speech but would be wrong on the GMAT. Here's an improved version:
they invited people such as I, who successfully have taken the online HBX Business CORe

2) I like a challenge, and remembered me buying a GMAT prep tool a quarter ago, so during the webinar I considered to give it a (moon) shot.
Improved
I like a challenge, and remembered buying a GMAT prep tool a quarter ago, so during the webinar I considered giving it a (moon) shot

Now, as for your question, i have a few things to say. You holding open the idea of applying in round three, so this doesn't give you much time to study. I don't know what you baseline is, but for a big improvement, it will take some time.

I would recommend taking the Magoosh practice GMAT toward the beginning, as an initial assessment. Save the GMAT Prep tests for right before the exam, at the end of your studying. This is what we recommend in our study schedules.

Does all this make sense?
Mike