Are you doing this with hashtags or something else? If more than hashtag, please take a look at something I came up with this weekend for labeling posts…
https://github.com/s3x-jay/nostr-nips/blob/master/68.md
Once I get nostr:npub1wmr34t36fy03m8hvgl96zl3znndyzyaqhwmwdtshwmtkg03fetaqhjg240's feedback, we'll put it up as a PR. But the "unstructured labeling" section would handle the use case you're talking about. Would even let people label things after the fact if they forgot to label it when they posted it.