I don't think that's a good characterisation of serotonin's role in leadership: https://www.forbes.com/2010/10/07/brain-science-neurobiology-leadership-managing-hormones.html
I see our most important ethical theories as founded in intersubjectivity. Kant's categorical imperative. Rawl's theory of justice. AO Wilson's development of our understanding of eusociality.
And not just in ethics. In the Buddhist metaphor for dependent origination, sunyata, and interbeing Indra's Net. And the idea of peer-to-peer reality. Objectivity is just reified intersubjectivity.
Like that, the Private Language argument can be understood and set in context: language is a kind of shared intelligence, which imposes salience landscapes on how we engage with the world that develop and extend our understanding of the differences between things participatorily, from whether blue and green are different, to understanding why qualia are not a meaningful idea (details here).
"either drowning in detail they cannot accommodate or aspiring for a
sense of order through abstraction they cannot attain"
This is about having efficient and effective salience landscapes, to facilitate cognitive grip. I like Vervaeke on this, and recovering our understanding of the role and importance of wisdom. When we have a good model of our self and the world, a heurustic explanatory overlay, it makes things tractable: chemistry forms units of explanation reducible to particle physics but more efficient in it's area, and in biology there is the 'unreasonable ineffectiveness of mathematics' exactly because this field is about systems that amplify the complexity of systems beyond that of chemistry alone (ie. it's systems that harvest Gibbs free energy to maintain local complexity).
Also multi-level selection, and game-theory applied to ethics, make sense of the non-individuality of ethics. And the intersubjective picture makes sense of Deleuze's virtuality, and interdiscursivity in general framed as translating between the realities of communities embodied in different modes of life (salience landscapes).
So I look at Singer's 'expanding the circle of moral concern' as a way to act ethically which expands our reality, and invites more beings into participation with our reality-creating discourses. That is moral progress, which deepens our engagement with reality, in ways that extend and develop what it is to be.