I think changing from subgift to mysterygift wouldn’t be ideal, as that’d mean it’s a potentially breaking change with some gifts now falling through the cracks for apps not designed to listen for a mysterygift.
What may be a interesting way to do it could be to have the mysterygift message contain an id, which all subgifts can refer to, that way if there’s no reference then it’s a 1 off subgift, otherwise not only would the presence of a reference indicate it’s one of multiple mass gifts but you could even see which one spawned it in the case of multiple from the same user being done within a short space of time. I’m usually opposed to additional tags being added, but this would prevent it being a breaking change and gifts being missed, and add additional data that’s currently missing.