When buying new, choose wool, cotton, linen or hemp over polyester, nylon and acrylic, especially for fleece, knitwear and anything washed often. Buy less, buy second-hand and wear things out.
The PLOS One laundry study found natural fibres shed around 165 mg per kg per wash, comparable to some synthetics, but keratin degrades in weeks to months and cellulose in months to years, while synthetics persist. Blends shed more than pure fabrics.
The catch: cotton is water and pesticide intensive and sheep produce methane, so the win is about persistence, not zero impact; a second-hand polyester jumper still beats a new cotton one on most measures.