Capitalism itself is one of the most unethical systems we perpetuate.

Capitalism is ethically neutral. I wholly endorse your attack on "fast fashion," or any other shopping that supports behavior we should not condone. But if it is ethical to buy cheap goods because you're poor, then it is ethical to create cheap goods for poor people to consume.

Capitalism is what the demand side demands it be. The solution to poor people buying goods made exploitatively is to reduce the number of poor people and then ban the production in question. (Banning is just a politically mediated boycott.) If consumers reward ethical businesses and punish unethical businesses, capitalism will become "ethical." It's just a pathway from demand to supply; it does not care who travels it.