If changing random_page_cost from 4 to 2 makes a difference, then probably there are no good indexes. The choice between Seq Scan and Index Scan should be obvio

If changing random_page_cost from 4 to 2 makes a difference, then probably there are no good indexes. The choice between Seq Scan and Index Scan should be obvious without depending on small adjustments or one day, with slightly different data distribution the plan will flip to a bad one