Sustainable fashion marketing focuses on materials (organic cotton, recycled polyester, deadstock). The actual highest-impact sustainability practice is wearing fewer pieces more often — buying 5 quality pieces worn 100 times each is more sustainable than 50 'sustainable' pieces worn 10 times each.
What this looks like
Smaller wardrobe. Repeated outfits. Repair before replace. Vintage and second-hand for new additions. Skip the constant churn that 'sustainable fashion' brands also depend on for revenue.
Buy less; wear more. The most sustainable garment is the one you already own and continue to wear.