Is there a way to tell Postgres “please don’t use index X when queries that could use index Y instead occur?”
No. But you could re-write the query to make the date index useless. The simplest way that comes to mind is putting the query that does your full-text search in a CTE (WITH keyword, it is an optimization boundary) and then ordering and applying your limit to the materialized set that comes out of that. eg.
WITH cte_full_text_results AS(
SELECT date, result FROM big_a_table WHERE text = 'whatever'
)
SELECT * FROM cte_full_text_results ORDER BY date DESC limit 10;