We probably have different ideas of "paying what you can". How do you judge whether someone is only paying what they can, or they are taking advantage? Not a rhetorical question.

One way is like we do today -- highly-paid positions often come with high-quality insurance, which is (ime) fairly expensive and low-paid positions have less expensive insurance that is a bit worse (all this on average, I'm sure there are outliers).

We are in different pools today but I don't see how that's meaningfully different than everyone being in one pool if the individual pools are large enough and you can't discriminate the population of the pools.

There are cases where it’s hard to tell if someone is truly paying what they can, but there are also cases where it’s easy: when it costs more than they can pay. Health insurance trivially fails the “paying what you can” criterion for sufficiently poor people.