if you see something which is worthy of a downvote: down vote and move on! Don't engage with it and feed the algorithm/engament machine so other people are exposed to it when sorting by active.
Disagree. You should politely state why you disagree. Engagement is good for newer websites like lemmy and you don't need to be rude or combative to disagree. One of my issues with reddit is when people would get downvoted for making a fair point or observation.
Doubt threads will even use activitypub. Tumblr said it was going to months ago. Never happened.