Equality does not somehow stem from rational agency into an observable and measurable feature. Even the first premise of ethics seems too difficult to justify.

It requires sort of argument from tradition or ideal theory so that we start from that people already accept atleast the baseline equality as non aggression and equal right to decide about other rules.

That works as long as people agree on them.

But another way Rawls uses is that all knowledge is a communal thing by definition. Science is valid only if the community agrees on the theory at hand. So ethics can be comparable science if there is a viewpoint that detaches from the aposteriori to apriorism that fits the idea of inductive logic (although Rawls speaks paradoxically of aposteriori apriori which he explains away with the ideal theory). And everyone who can do this has an equal vote on ethics, like science has its criterion (although it can lead to fallacy of expertise) So there is no many ethical theories, only peoole who have the virtue for ethics and who don’t.