I would say the biggest stage Mint has been promoted for and performed on (so not including AX cause she was not promoted)
>1. ANYC
powergap
>2. FantomeThief
powergap
>3. Fantastic Reality
The keyword to the phrase "the big stage" is significance. ANYC is the most significant convention main stage you can get on. Only Holollive has gotten there (until somehow Phase did but that will never happen again)