However I am specifically a proponent of voluntary collectives for things, as government forced cooperation seems suboptimal and fragile to me.
Isn't this the exact thing insurance solves, in particular catastrophic coverage insurance? Nobody really gets "ruined" by paying out of pocket for one X-ray and two aspirin.
The problem is that the insurance is so expensive some people can't afford it. But the only sense in which single payer would "fix" the high cost/overhead/waste problem is by de facto regulating prices, which can be done even without it but which has a lot of obvious problems -- if the regulator chooses too high a price then it's still wasteful/unaffordable but too low and there will be no providers (or the providers will sacrifice quality to hit the regulated price).
What we need is a real solution to cost disease, which probably has something to do with reducing the regulatory compliance costs so there will be less overhead and more viable competing providers, requiring price transparency from providers, and then having people pay out of pocket for all non-catastrophic care so the patient has the incentive to decline treatment or find a less expensive provider when the treatment is unnecessary or overpriced.
This is why there are regulations to ensure a minimum standard of care. Above that, the provider is the one who has to convince the patient that their service provides some benefit over the lowest cost provider.
And isn't voluntary collective healthcare just another way to describe private insurance companies, especially HMO's?