I don't think it's true that if a company is successful then you will make them two dollars for every dollar you get. I think many large companies are profitable due to rent-seeking or externalizing costs, and the marginal gain from the N-thousandth employee is negative, but they can't lay off employees or even stop hiring because of legal or political reasons. If you get a job with one of these companies, then you are wasting their money, which is a good thing to do against an evil company.

Noam Chomsky has been repeatedly asked whether capitalism is a good thing after all because the average quality of life has improved under it. His reply is that average quality of life improved under slavery too, but that's not a justification for slavery. You can see him discuss this at https://www.youtube.com/watch?v=W6QAqU2KpaY.

You and I are having the reverse conversation, which is whether it's better to participate in a harmful system for the sake of making any progress, or better to opt out entirely. Actually, because humanity is currently headed off a cliff due to climate change and the worldwide rise of authoritarian governments, the choice is between participating in the harmful system or allowing large-scale catastrophe.

I agree that libraries fit the criteria I laid out, but I guess that like other do-good organizations, they are poorly funded and have a glut of trained specialists and willing volunteers. It's a sign of our dysfunctional times that the more directly helpful an organization or job is, the less well funded it will be.