A voluntary society cannot be designed at all. It will be emergent. When a critical mass of people realize that the rules we tell children to live by (namely don't hurt people, don't mess with their stuff, and keep your promises) should be applied across the board, and that no other general rules are necessary, then such a society will form.
There can be no formula for dealing with people in need. As soon as such a formula is known, most of the marginally needy and some of the non-needy attempt to game the system. Subsidizing poverty creates more poverty. The best way to deal with those in need is on an individual case-by-case basis. It's too important of a problem to be left to monopolists. Concrete answers are wrong answers.
Killing and stealing only works until the productive people stop producing, and then everybody starves. The productive people started leaving South Africa in droves when the anti-capitalist Nelson Mendela took over. There's no place on earth with more natural resources per acre than South Africa. If people are starving there, then it's because the government killers and thieves created an environment hostile to peaceful trade.
Actual good rebuttal, but this assumes all will be rational and well-adjusted. The killers and stealers won't think like this (or won't care/won't have the skills needed to make it in the world), and people who refuse to live by the sword will not be able to allow themselves to starve if they can help it. If we both turn out to be right, you about killing and stealing losing efficacy over time, and me about killers and stealers doing killing and stealing anyway, that's a potential huge blow for your ideal. Having your reasonable people inevitably starved to death at the hand of greedy murderers and thieves is a likely death knell.
Personally, I'd rather people game the system by collecting more food stamps than they are legally allowed, rather then having them just straight up try to blow my brains out and take all my stuff. While I wish we could deal with them on a case by case basis, under the current system I think that would cost more than the money saved by catching fraud. If you think the ability and cost-effectiveness of doing this would be improved in your ideal world, or even if you think there is a way to improve it
under the current system, I'd be very interested in hearing about that.
Unfortunately I don't think meaningful improvement is possible under the current system. This is one reason why I am a revolutionary. I see a fundamental weakness in monopoly government that cannot be corrected without allowing distributed competitive governance.
There is no easy solution to the problems in South Africa. I see the best case scenario a hopefully temporary reversion to tribalism. If I lived there, I would leave if I could and retreat to an area controlled by my tribe if I couldn't, hunker down and ride out the storm. It is likely to get much much worse before it gets better.