I guess what I don't understand is if the diy- and "self-sufficient small communities"-aspect of solarpunk is coming from "we have to start somewhere" or "that is a good overall principle for the society". Would be glad if you could point me in some direction there.
Speaking for my self, I think it's a bit of both.
There is certainly a big aspect for "we have to start somewhere", because we do. Even if we were the ones who decided how the world was organized, and could unilaterally order everyone, "hey we gotta live efficiently within the planet's resources" we'd still need to carefully roll things out one step at a time. Also, because we don't even know what will work well yet! So we need to experiment, and it's way easier to do a small scale experiment!
Secondly, I think there is an aspect of "good overall principle" at least, there is something in that sort of lifestyle that is intrinsically appealing. But, on the other hand, again, smallness gives the freedom to experiment. I also don't think you should take this too literally: instead take it metaphorically: it's about the social relations that hold society together, and that being explicitly village like, or something like that. So I think we are attracted to the idea of solarpunk because we want something different. But this too is a have-to-start-somewhere, really, because a community of radicals who want to do everything differently, will become very different when lots of people start to get on board.
But the idea that everyone on earth should live in a little village and grow their own food, (anarchoprimitivist model) I'd say: No. I'm pretty sure no one waving the "solarpunk" flag advocates that as the singular solution to the world's problems (especially since solarpunk is explicitly tech-positive), although that's not to say that there are not people who do want to live like that.