Forgive me, the focus you put on 'platforms v publishers' is reminiscent of the uninformed section 230 discussions happening everywhere right now, which are exhausting.
Afaict section 230 isn't a special provision granted by society, it is a legal clarification of existing rights of speech & assembly that has the effect of limiting certain types of frivolous lawsuits. Clarifications seem materially different than government granting a corporation a local monopoly. The latter should definitely have strings attached, but the former?
Now you could argue that the basis for the decision to grant utilities local monopolies is rooted in similar natural monopoly circumstances (economies of scale and resource limitations) as that of mainstream social networks (physical infra & space in former, network effects in latter), and I'd probably agree, but that would be an entirely different focus than section 230 reform.
You're also advocating for a very black & white solution (platform v publisher, based on a choice about moderation), but there's a lot of specifics lacking such that its hard to determine if that's even a good idea. My immediate reaction is: online communities are extremely varied in both operation and medium (design, algorithms, what 'speech' even is, etc), and anyone who has had the experience of building and operating communities would see 'all legal speech should be published' as frankly a laughable premise. It raises far more questions than it answers, and likely just kicks the hard work of how products are designed over to lawyers who will definitely get it wrong.
My hunch is we're barking up the wrong tree with 230, and the underlying issue is too much power concentrated in too few entities. They have too much control over too many people's experiences, and when they use that power people confuse it with censorship. If we focused on the issue of natural monopolies arising from network effects, we might come to very different solutions, like decentralization with standard protocols for social graph interoperability.