I have searched a lot about this question, there are no concrete answers to this. I have a AWS Redshift DB, has around 6-7 schema' with 10-12 tables in each. and dashboards are made within schema level as well as across schema.
here's the use case:
I have seen a thread saying that it's possible to give access to schema but they haven't mentioned that How.
https://github.com/apache/incubator-superset/issues/5483#issuecomment-494227986
As per the Superset documentation, you can not create access level on the schema but you can create access on data source level. Or you can create custom data sources and can create desired roles as per your need.
Refer: https://superset.incubator.apache.org/security.html#managing-gamma-per-data-source-access