What I’ve noticed about Nostr hashtags, having never used social media with hashtags before:

It’s a little difficult to predict when the more common form of a hashtag will end in -str; my only guess is probably if it’s likely to be a common part of home life, e.g., dogstr, catstr, and foodstr

Tunestr used to be used for music generally, but now it’s an actual thing, tunestr.io

Several ending in -chain are related to meals, like coffeechain and breakfastchain.

Grownostr is meant to be used for anything at all NOT related to Bitcoin — that’s what I was told, at least.