With a night's sleep and a day's break, I tried the following. I have removed the line 'has_archive' => true in the code for the newsletter cpt

This causes the admin list of newsletters to actually be newsletters and not the list of blog posts.

Then I checked the feed at domain/feed/?post_type=newsletters and it actually gives the newsletter rss. (I had previously been trying domain/newsletters/feed which rendered the regular blog posts with 'has_archive' => true and no posts at all with it set to false)

Now I have a second rss feed that renders the correct content, and the list of newsletters posts is actually newsletters.

I don't know if this is the best solution, but it is a working solution.
Thanks to those who have contributed and I will check back in case someone has a better answer