I agree with you that pressuring people to change individual purchasing decisions is not a good strategy, or a good use of someone's time, and that there are better approaches to reach the same goal. My point is that you're taking the factors that limit a person's ability to exert their consumer choices, and making a leap to say that the consumer has no material impact on the world. That doesn't make much difference for the moral implications of this issue, but it's anti-materialist, and it gets in the way of understanding how the world operates.
The chocolate industry has built a machine of cruelty that controls governments, and has continued for hundreds of years despite all the people it has immiserated and motivated to fight against it. No one is going to take it down by putting a candy bar back on the shelf at the grocery store. And yet, the industry is the size that it is, and no larger. If the capitalists had their way, it would be ten times bigger, and then ten times bigger next year. It's limited by material factors, including the fact that people will only buy so much of the finished product. Marketing, lobbying, mercenaries, etc. are all tools that companies use to push against those limits, but the fact is even someone who doesn't give a shit about labor practices, and puts five cases of chocolate bars in their cart before deciding they only really need four, is participating in limiting how many child laborers cacao companies are capable of exploiting.
The impact that any one person, or the handful of people one person could recruit, can have on overall consumption is very small compared to the global population, but those minuscule changes do propagate through the system of production and exchanges. You can't be a communist and not believe that tiny contributions can add up to meaningful change. Organizing and coordinating those contributions is far more important than any one individual, and it's good to emphasize that and redirect to it, but that can't be done by dismissing that the small changes matter at all.
You make a good case for why the focus shouldn't be on moralizing specific consumer choices, and why the products someone buys says little about them in comparison to the impact they could have in other areas. You're going to have to rely on the strength of those arguments because, while it would be nice if we could fully absolve ourselves by saying that the commodities we buy have exactly zero impact on the world, it isn't true.