Take a deep breath -- it's going to be fine! Your profile is pretty strong in my opinion, and I think with a little work, you could overcome your low GMAT score. Still want to pick a breadth of schools to apply to to be safe, but you've got a shot.
Firstly, your professional experience is pretty interesting and it seems like you'll have a lot of leadership and team stories to share. Your leadership also stands out in your entrepreneurial experience and you will have excellent stories to tell from that, too! Off to a strong start!
Academically, your GPA is definitely low, as you've already pointed out. I don't think you need to go back to undergrad, but I do think it would help to enroll in a couple of b-school type courses (stats, econ, accounting, finance) and attempt to get A grades in those courses (1-2 courses should suffice). Ideally you could take these at an accredited university (in person is better than online if possible!). Also be sure to have your recommenders highlight your analytic capability. Finally, address your GPA in an optional statement - take responsibility for the fact that you lacked the maturity to take your studies seriously in college and that you had a hard time adjusting, etc. If there's an upward trend in your GPA, point that out. And point to your GMAT scores and grades in the more recent courses as proof points that you've matured and that you are academically capable.
Extracurricular - between the start-up experience and volunteer experience, plus your activities from college, seems like you are a pretty active and engaged individual. If you have any hobbies that you are passionate or especially accomplished in, you should highlight those too to add some personality to the profile.
All in all, I think if you achieve that GMAT score between 720-780 and take some supplementary courses, the foundations for a strong profile will be there. Then it's just a matter of strong execution - choosing the right stories, conveying all the right things, showing breadth and depth of who you are, etc.
Your school list is well thought out and I would choose a couple in each tier that you've come up with to apply to.
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