Alright, this might be a controversial take, but I don't think it's really about the flower part at all, weirdly enough. In 'Demon Slayer', Shinobu's whole deal is poison. She uses it because she's physically weaker and can't behead demons. Her fighting style is all about speed and precision jabs with that needle-like sword, which is literally called "insect breathing" and not flower breathing.
So when fans talk about a "flower hashira," they're usually mixing up the motif with the role. Kanae Kocho, Shinobu's sister, was the Flower Hashira, and her style was graceful and flowing, but we barely see it. Honestly, the unique skill is thematic—representing transience and beauty, maybe with petal-like sword swings. But in practical terms, a flower hashira would likely focus on deceptive, beautiful movements that hide lethal intent, less brute force and more artistry. It's a shame we never got a proper showcase.
The fandom kind of fills in the blanks with OCs, giving them powers over plants or perfume-based attacks, which is cool but pure headcanon.