This is sort of one of the reasons I think a price floor of some sort might help.

It would put a minimum limit on how much vertical integration could benefit.

E.g. if somehow we say a t-shirt needs to cost at least $30, then Amazon can’t sell them for $5, so the small company who makes tshirts can compete with them.

I don’t know exactly how you could sensibly implement that - there’d be a myriad of ways to bypass it depending on how the law is phrased - but just a thought.