I try to avoid supporting unethical brands by buying almost everything second hand. Not just clothing but furniture, electrical items, technology (laptop and phone).
Companies base their policies on how much and what they are selling, and at what price. So every time you buy a brand new item, you are giving them data which will affect their future sourcing of goods. So if Shein, Temu etc. are selling lots of clothes made in slavery-like conditions, they will continue to source them from there as their prime motive (like most businesses) is to make a profit.
Buying second hand doesn't 'feed' this, even if the actual items were made in China, are Shein branded etc.