The old myths would be relevant historically, but not phenomenologically or existentially. The problem is that, like food, art can go stale. The liberal bishop John Spong made this point. He thought religions should update their myths to speak to new generations. The danger of doing so is immense, of course, because great myths and other artworks are the result of genius, not of committee compromises. Still, the downside of not attempting to rethink our guiding stories is that we may be held captive to outdated ideas.
There are surely exceptions in which old artworks still speak to us as proto-modern or as virtually modern. Job is a powerful story, but the background is nonsensical (since it's theistic), and indeed the lesson is crypto-atheistic. The Jewish theological framework was in the process of overcoming itself, as Jews dedicated themselves to secular progressive pursuits, leaving the afterlife to take care of itself. So is that scaffolding for the new Jewish project relevant to fully secular modernity? The vestiges of theism taint the ancient art's merit.
In The Intellectual Adventure of Ancient Man, Henri Frankfort argued that religious myths are functionally phenomenological. They're not about getting the empirical facts right, but about describing how life and the world seem from a human perspective. The best myths have that universal scope, which makes them "existential," but the lesser myths speak mainly to tribes and to the politics of the day. Of course, if you look hard enough, you can find some merit even in mediocre art. But dealing with great art would be better.
That's an interesting point about the potential for tension between cosmicism and humanism. Certainly, there's no guarantee of consistency since I'm trying to combine different philosophies and standpoint that strike me as important.
I'm ambivalent about secular humanism, though, so my criticisms of it would stem from the darker side of my worldview, from cosmicism, existentialism, pantheism, and so on. For instance, is technoscientific progress a good thing? Yes, if it leads to the sci-fi paradise of transhumans. No, if it wipes out life on this planet.
Lovecraft doesn't really have much of a philosophy, as far as I'm aware. He's like David Hume in that he took scientific assumptions to unpleasant, extreme conclusions. Lovecraft thought that naturalism and scientific progress are tragic in that scientists are bound to undermine their humanistic pride by discovering the alienness of nature. This is a problem for modernity. John Vervaeke calls it the meaning crisis. And my neo-Nietzschean solution is that we could use a modern mythos that shows us what honour is even in that blackest of scenarios.
