I was on Twitter since 2010 and I did not share your experience of seeing it as a toxic place. This could be due to the fact that I curated my follows carefully. To this day, I can go back to see what the design or startup community talks about and I do not see any toxicity. I do see the toxicity in the Bitcoiner tweets and anything trending related (usually politics).
Nostr already has communities, though imperfect, and people can create very narrow discussions based on a specific topic. We don't need to guard nostr in any special way from anyone because they can always create their own community or curate their own feed. We have the benefit of no algo, and eventually algo choice so rage-inducing content doesn't stand the same chance of making it to the top.
But, generally speaking, the more people join, the more varying opinions you'll see and someone will naturally start posting negative crap. This is true of anywhere. I think nostr is better positioned to handle this type of activity because it does not reward engagement in the same way. You are more likely to get blocked than your note "go viral". It may land into trending, but won't last very long.