Kudos for remembering TeXmacs, for some reason it seems to be seldomly know outside of math-heavy STEM circles although it is a Scheme-programmable structured tree editor worth checking out for general document authoring.
TeXmacs does technical typesetting with similar or better quality than LaTeX (and it does not use LaTeX behind the scenes for typesetting, unlike LyX) and keeps a live representation of the document as a tree that you can query and modify (e.g. using PEGs). It is also fully WYSIWYG (unlike, say, Typst).
Using the graphical mode one can freely position math and images and text on slides with perfect typesetting. And the switch-and-fold environments let you build trees of visibility for arbitrary parts of the document (the most obvious application being having exercises that can be switched to show the solutions).
What is more: in TeXmacs one can be enforce a certain degree (to the extent permitted by the ambiguity in mathematical notation) of semantics for math, so that good-looking expressions can be directly sent to a computer algebra system (CAS) for evaluation (and printed back in a way you can read).
So TeXmacs can interface to CASs and in general any programming language.
In fact, thinking about code editing, am not sure why TeXmacs could not, in principle, be the Scheme-based Emacs that some people have dreamt of (cf. the defunct Guilemacs effort, https://news.ycombinator.com/item?id=23304457). Maybe it will: a dedicated community of hackers led by Darcy is doing experiments in this direction in a friendly fork under the name “Mogan” - https://mogan.app/guide/what-is-mogan.html, although I have no idea what are the plans for Mogan Code.
TeXmacs does not use Electron, so while it does import HTML reasonably, it won't render websites in their full glory. But there was a recent job offer for somebody to work on improving the rudimentary capabilities for collaborative editing, so it might end up offering an alternative to Overleaf.