Because the community on Lemmy is so much smaller it's a lot easier for small groups of dedicated posters to dominate discussions on certain topics.
I've noticed a lot of the same behaviour as you have on certain topics. Unfortunately it's difficult because like you say engaging on those topics is frustrating because the people with an agenda have more time and energy than you to dedicate to pushing their narrative, and aren't open to more nuanced discussion.
There's an interesting blog I think about regularly about online communities that I think you might find interesting: https://eev.ee/blog/2016/07/22/on-a-technicality/
Now that article calls for banning of assholes. I don't think that'll work on lemmy, so instead I propose this: If you just accept that those people are going to continue to do their thing and instead engage in the more positive parts of Lemmy then overall we might be able to build a bigger community of people who add positively to Lemmy. If you or others who are being pushed away leave then the asshole : positive people ratio will only get worse.