PostgreSQL is over 90GB, SOLR is under 1GB. The PostgreSQL has to handle writes, reports, etc..., so I doubt it will cache as efficiently as full-text engine,

PostgreSQL is over 90GB, SOLR is under 1GB. The PostgreSQL has to handle writes, reports, etc..., so I doubt it will cache as efficiently as full-text engine, you'll need to have full or partial replicas to distribute the load. And, yes, I agree, almost all of this can be done with separate search table(s), but this table(s) will still live in a 'crowded house', so again replicas will be probably necessary at some point. And using replicas brings new set of problems and costs ;-) One client used MySQL for fulltext search, it was a single beefy RDS server, costing well over $1k per month and the costs kept raising. It was replaced with a single ~$100 EC2 machine running Meilisearch.