How does befriending the neighbors, rationalist or not, fit on this scale? This reduces monoculture, opening everyone in the neighborhood to more perspectives. It saves time planning, too. You just go around the neighborhood and see what happens. Maybe you do scoping out beforehand to find a good place to live. That could take a while, but sounds like a lot less work than designing an intentional community.
Whether some of your desiderata are fulfilled depends a lot on trust. Kids can hang out at neighbors' houses, if the neighbors are trustworthy. There may or may not be missing stairs -- it's a lot harder to filter beforehand, but on the flip side, maybe it's easier to filter once you get there, because you're not so pressured to relate to neighbors. Proximity is an issue, though.
Some of these desiderata seem hard to get fully with the "just go into a neighborhood" solution. People might have each other over for dinner, but it's not the same as just pooling a bunch of food.
Like any strategy, befriending neighbors still carries the risk, which many have discussed so far, of hanging around people who want to exploit you. Does mingling with the neighborhood expose you to exploiters more than hanging around in a community? One reason to expect it might is that people in the neighborhood are more likely to just have different morals than you. On the other hand, a predator could more easily target a monoculture because one tactic is more likely to work on all of them. Also, a predator sounds much more devastating to a close-knit community than a loose-knit neighborhood.
Rationalists like to live in group houses. We are also as a subculture moving more and more into a child-having phase of our lives. These things don't cooperate super well - I live in a four bedroom house because we like having roommates and guests, but if we have three kids and don't make them share we will in a few years have no spare rooms at all. This is frustrating in part because amenable roommates are incredibly useful as alloparents if you value things like "going to the bathroom unaccompanied" and "eating food without being screamed at", neither of which are reasonable "get a friend to drive for ten minutes to spell me" situations. Meanwhile there are also people we like living around who don't want to cohabit with a small child, which is completely reasonable, small children are not for everyone.
For this and other complaints ("househunting sucks", "I can't drive and need private space but want friends accessible", whatever) the ideal solution seems to be somewhere along the spectrum between "a street with a lot of rationalists living on it" (no rationalist-friendly entity controls all those houses and it's easy for minor fluctuations to wreck the intentional community thing) and "a dorm" (sorta hard to get access to those once you're out of college, usually not enough kitchens or space for adult life). There's a name for a thing halfway between those, at least in German - "baugruppe" - buuuuut this would require community or sympathetic-individual control of a space and the money to convert it if it's not already baugruppe-shaped.
Maybe if I complain about this in public a millionaire will step forward or we'll be able to come up with a coherent enough vision to crowdfund it or something. I think there is easily enough demand for a couple of ten-to-twenty-adult baugruppen (one in the east bay and one in the south bay) or even more/larger, if the structures materialized. Here are some bulleted lists.
Desiderata:
Obstacles:
Please share this wherever rationalists may be looking; it's definitely the sort of thing better done with more eyes on it.