You can try TeXmacs in your browser at https://yufeng-shen.github.io/Mogan.html . (It's actually from a fork of TeXmacs called Mogan, of which I've been a happy user due to better CJK support.)

By the way, I do think TeXmacs is an Emacsen as it provides Guile/Scheme as an extension language, though I don't know how customizable it is. (I think the built-in REPLs for Python/Maxima/Scheme/... are written in Scheme.) And then, it does support quite some TeX commands (and you input them by pressing backslash followed by their command name), so I do think their "TeXmacs" name is very much justified.

Early on in my computing life, I discovered TeXmacs as a user interface for a Computer Algebra System I had been playing with called Axiom. Ironically, this was before I had ever even heard of either TeX or Emacs! It seemed like a cool piece of software, but when I later learned LaTeX I discovered I prefer non-WYSIWYG for everything but lecture notes. Still, in the years since I've recognized that this setup, combining a math engine with a rich display interface, was an early version of what would later be popularized as Notebooks.

Are there any „real world users” of this? During all my years in academia I haven’t met any. Most just use plain LaTeX. Some do MS Word. Rarely something else. Never Texmacs. This is my experience at least.

With stuff like Overleaf and plugins for modern IDEs, honestly I can’t say LaTeX is a bad experience. It does what it should.

There is also a fork of TeXmacs called Mogan https://github.com/MoganLab/mogan

The name is TeXmacs - but "Notice that TeXmacs is not based on TeX/LaTeX." I wonder why they chose that name.

Such a weird project, starting with the weird name that sets all kinds of wrong expectations

I love TeXmacs so much I just use it as a regular word processor