But I noticed, none of those interactions ever turned into further interactions outside, let alone friendships or things like christmas dinners.
I've also co-run a community centre (a hackerspace) for years, and been one of the more prominent members that everyone meets at some point, which I'd say is a pretty social thing to do. Those too also didn't turn into friendships outside, but perhaps the element of formality in the relationships didn't help with that.
I think your approach to housewarming is a great idea. Even if knocking on neighbours doors feels like an oddity these days, and nobody else seems to be doing it, as a brand new neighbour that's a great time to do so, inviting people to a party that's low effort for them.