It’s been reasonably active here definitely not reddit in it’s prime, I haven’t found it worthwhile to self host lemmy but others may chime in that do. I’ve got a few other things I’m hosting on my Dell r720xd matrix, jellyfin, wireguard, next cloud, gonic and other minor things.
Opensearch will be the most performant. Anything sql will likely start to stumble with lots of stories or really long stories where this is exactly what lucene based search engines (solr, elastic, opensearch) are designed to do. Could an SQL solution solve your problem, yes, but it may be a bit on the slow side as your amount of stories and size grows.