We have considered various integrations including sphinx, melli, elastic, solr/lucene but they come at a cost. Hosting another process to run indexing, risking out-of-date indexes, complexity… etc all are not free.
I would like to see how much mileage we get out of PG prior to exploring any other options and keep them as a last resort.
Very interesting problem, yes, they are (and always have been) de-prioritized. I think at a minimum we can look at adding a site setting to discourse-solved to allow admins to decide what to do in these cases (prioritize/deprioritize/neutral etc.)