First, thanks for such an informative essay on the most important topic we humans must address: how we should live our lives.

If I may, my interpretation is that existentialism arose as one part of the 'next wave' in Western philosophy following 'the Enlightenment' (which had succeeded the sacrally oriented thinkers of the Middle Ages). The precepts and premises of 'the Enlightenment' were successfully questioned by David Hume, resulting in attempts to address the issues he raised: thus, that "next wave."

Existentialism (presaging Postmodernism) accepted that there could be no coherent, given conceptual framework--ideological or theological--to predetermine how we should live our lives. That is something each individual must undertake to discern for oneself (though there is of course among all philosophers a sense that everyone should arrive at the place at which that person had arrived concerning these matters).

The thing is, we humans live together in groups, which imposes upon us the issue of governance. In that context we simply cannot leave it to each individual to determine how one should conduct oneself in society.

The best we can do in that regard is the most just governance possible. What we need for that is an ethic of justice--a rule to govern governance--that all people can know is undeniably applicable to all people. That requires an ethic that is located in whole--its determinant and its referents--in material existence.

I have developed such an ethic. If curious, "Can't get Any Simpler" is a "2 min read" here in Medium with links to more essays on the topic--with nothing I publish here behind the paywall (given that the perhaps pennies I would expect to earn would have no impact on my life, anyway).