The only two things she'd be able to do in Holo that she can't do as an indie is perform at Holofes every March; and the EN concert every August, if she were to join Holo EN instead of Holo JP or DEV_IS (she's NSL, but fluent enough in Nihongo to join one of the two JP-language branches)
But spoiler alert - she's not gonna join Cover without getting some sort of "you can continue being Mint Fantome and also be allowed to generally do your own thing to some extent" deal similar to what Suisei got back in the day. If Mint got such a sweetheart deal, it would probably also include the "not joining Hololive proper" part that Suisei's contract initially had

>>102366439
>>102366904
Picrel, for both of you but more so for 439-san