In these categories (secularists, humanists) and in the two names you mentioned (i've left out freethinkers), I think there is a key problem in that there is not yet a full and dramatic separation from the "virtue ethics" approach that's probably close to the root of Stoicism. For example I have a lot of appreciation for Catherine Wilson but I think her books fail to draw a distinction between her own personal social preferences (which we all have) from the philosophical underpinnings. Every time we represent our own ethical choices to be "the Epicurean view" on an ethical controversy, I think we bury the ultimate point deeper -- that there is no basis in the philosophy for representing that our own choices are the "correct" one. In that I think each of the categories are adopting the Stoic "one size fits all" approach, and that's deadly for the contextual and sensation-based roots of Epicurean thinking. In my view they are essentially Stoics, just taking the position that their own view of what's pleasing to them should be adopted by everyone as part of the philosophy. I'm no expert on Kant but wasn't that his view -- to be valid a position has to be extensible to everyone everywhere all the time?
I think to be philosophically consistent you have to do both -- affirm (1) that you understand that there is no single "good" for everyone, and your choices are no more justified by gods or idealism than anyone else's, while at the same time (2) asserting that your and your friends who see things the same way are going to pursue, to the best of your ability, your own version of the best way of life independently from those who see things differently.
I suppose a natural audience would be secularists, humanists, and freethinkers.
It's so frustrating because one would think exactly that, but in my experience those groups have been no better than average, or maybe possibly worse as target audiences. Possibly in large part because many who have joined those camps have done so more due to their rejection of establishment morality as a personal preference rather than because they recognized that there is no idealist or religious basis for the establishment view. Either they are into virtue ethics and remaking the world in their own vision, or they are rebels without a cause, rejecting Epicurean efforts at systematic thought as much as they reject any other.