It’s the MusicBrainz concept that makes the most sense for start date:
Begin date represents the date (in real life) when the character concept was created.
I’d strike out the “fictitious” linkage and terminology, as it’s not relevant. (For the same reason we never added a “fictitious” flag to Groups, because it’s not relevant.)
It’s really the same problem as solo projects. VTubers, like solo projects, absolutely have start and end dates if you apply artist intent.
In the same way solo project start and end daters should not reflect a date of birth and date of death if artist intent is considered, and instead use start and end dates of the project as if it were a group - non-indie VTuber start and end dates are the same, from what I understand. Unlike with characters in traditional media, a “graduated”/“retired” (ended) non-indie VTuber should never record new music.
Nobody bats an eye when we say a character designer designed a VTuber character.
(The 2 largest VTuber agencies use the terminology “talent”, by the way.)