Other than creating a functional index on the keys that you most want to access and include that function call in your query, I don't know that you can. Functional indexes get statistics while partial indexes do not. If a GIN index using path operator collected stats on the frequency of the keys, then you would be all set I think. Perhaps that is a feature that could be implemented.
Others will chime in I expect, but I tend to avoid using jsonb for any values that I need to use to significant filter a result set (ON and WHERE clauses).