Let me tell you about a specific thing that I saw in a different community, that I thought was a good way to make the community more welcoming.
I was in a meetup community about D&D. There was a guy who did a great thing there: every four or five months, he would create a meetup called “Meet And Greet For Players And DMs”. You could show up to the meetup and talk about the specific game you wanted to play in (or run). You could meet other people who wanted to do the same thing, and you could trade contact information, and after the event you could send people messages: “hey, come over to my place and let’s play that game that we both want to play!”
This is a great way to forge intercommunity connections of the sort that you’re talking about. It’s also remarkably low-effort: the guy would say “yeah, we’re going to meet in X location”, and then he’d show up at X location to proctor the event and make sure everyone got a chance to speak, and that was all he had to do.
(Disclaimer: I’m not a member of the community you’re seeking to change, so my consent is not necessary to your plans.)
A lot of the themes I’m seeing here (“many people feel lonely”, “some newcomers feel unwelcome”, “some people are disdainful and dismissive”, and especially “culture of making sure your own needs are met”) remind me of the Geek Social Fallacies post. I can summarize the Geek Social Fallacies post as follows: “some people are jerks; if you encounter a jerk, you shouldn’t feel obliged by politeness or niceness to interact with them.”
This aspect of Raemon’s post seems directly in opposition to the GSF post: Raemon admits that some people are “disdainful and dismissive”, and I’m guessing he would admit that some of the people that are attracted to the rationality community are sometimes socially awkward, but he nonetheless asks how we can make the community more open and welcoming.
I think this post would be improved if it addressed this issue directly.