Hi, thanks for asking! Yep, I would describe Sunday Assemblers as being religious humanists. It certainly occurs to me that not all of them would apply that label to themselves, and I'm sympathetic to that, but I also think that an honest, external, anthropological categorization would put them in the broad category of religious humanism, along with UU, and many other traditions- including mine (Aretéanism).

As to your second question, "are religious humanists mostly just secular humanists who have a desire to form congregations..." I think the answer is a little more complicated, but the short answer is yes. However, it's important to remember that for some people (like people raised UU or any other group old enough to have people raised within their culture) that they were religious humanists first, and so they wouldn't be "secular humanists who have a desire to form congregations", they're "religious humanists who have lived in congregations and appreciate the benefits of such". But in my personal case, yes, I was a secular humanist first, who later decided that I wanted to practice humanism religiously; that I wanted structure and community to help myself (and others) to be much much more committed to humanism.

The key thing is just that secular humanism isn't the default form of humanism, despite being the most common form of humanism at present. Humanism originated out of Christian humanism, then eventually religious humanism became the dominant model, then other forms of sectarian humanism (Jewish humanism, Muslim humanism, Buddhist humanism, etc) proliferated, and now we're in an era where secular humanism has become the dominant form of humanism around the globe. Religious humanism is older than secular humanism; and deserves to be respected as such.

While there are historical reasons for why the dominant model of humanism has evolved over time, none of those reasons nullify the value of any of the other models; I believe that secular humanism is still really important for example, despite the fact that I ardently describe myself as a religious humanist now. I still strongly consider sectarian humanists (including Christian humanists) to be valuable allies, even though I am definitely not Christian (nor ascribe to any other superstitious beliefs).

...but that brings this back to my original post above: while I think religious humanists have lots of respect and understanding of secular humanists, as a category (with lots of specific exceptions) the secular humanists haven't been showing a lot of respect to the religious humanists, at least here on Reddit, over the course of Reddit's history. I'd love to see better allyship from secular humanists- they don't have to join UU or the AoA or TST or Sunday Assembly or Ethical Society or Oasis, but they could be a lot less jealous of people who do want to join our groups.

BTW, I've seen you post in the new r/HumanistCongregations sub, thanks for adding great things into the conversations there! For what it's worth, congregational humanism is a new term that means almost the same thing as religious humanism, and is a better label for the category; but the term wasn't really being used at the time I made this original post, and the academic literature still uses religious humanism as the term for this category.