With 428 marks in NEET and being from the ST category, your chances of securing a seat through All India Quota (AIQ) Round 2 depend on various factors, including cutoffs, category-wise availability of seats, and the number of applicants in your category. Historically, cutoffs for the ST category are relatively lower compared to other categories, giving you a better chance compared to general category candidates.

However, since you've mentioned there's no chance through Meghalaya state quota, your best hope lies in the AIQ. The exact cutoff varies each year based on the difficulty of the exam and seat availability, but typically for ST candidates, the AIQ cutoff can range from 400 to 500 marks, depending on the institution and state.

While waiting for the AIQ Round 2 results, it’s advisable to keep an eye on any mop-up rounds or seats available through private institutions, as these could provide additional opportunities if AIQ doesn't work out. Additionally, consider exploring deemed universities or private colleges, where your ST category may provide some advantages in terms of relaxation of cutoffs.

Stay optimistic and prepared for any further rounds, as AIQ often opens more doors in subsequent rounds.