I've tried everything to reach people like this with virtually no success.

I don't know how much of it can be WoW-refugees, it's been a long-standing topic of debate both on these forums and in the game in general. I once suggested to someone in a discord server that I was sure they could do some dps and that they just needed to have a little faith in themselves and was told to "not tell them what they're capable of" so... yeah, there's that.

SE has made the skill floor for players doing casual content so low that bosses like the Mist Dragon in The Burn have threads here and on Reddit calling for them to be nerfed and healers got the shortest end of that stick because in a party doing small pulls in most dungeons the healer isn't even necessary.

I don't think there's anything to really do for those people other than offer advice and go from there. If they're rude about it maybe kick them? I hate the idea of advocating kicking sprouts but some people only learn with tough love.