Is there any way i can change the schema name when ingestion a dataset to datahub like public.datahub to new_name.datahub

Not without modifying the current ingest code.

you could also write a custom transformer to change the name: https://datahubproject.io/docs/metadata-ingestion/transformers/

But isn't schema part of the dataset key at the point of creation?

The transformer has access to the entire dataset snapshot, so it can change the dataset key as well