I feel that people are on Lemmy because of their convictions and are more willing to boycott something they condone.
We can just keep posting motivating stuff and it will reach c/all. Showing the truth about the way we exploit animals will convince the ones who truly care about them.
Recently I saw another topic here with lots of comments from c/all and it was a shitshow, with lots of vegans calling people narcissists and generally putting people in a defensive position. That is something we should really avoid.
We should make compelling arguments so people can read them and change their minds about what they're supporting. Try to be thoughtful even if the other person is not. Lots of people will end up reading your comment anyway.
If you find someone who really don't care about animals, the best option is just not engage. Making a negative comment is awful and off-putting.