Please give org-mode and org-babel a try also. There is much more utility with org and a much larger user-base. Org is more of a standard than TeXmacs, which I haven’t heard of until today. You can do all of what TeXmacs is doing without using their interface, which looks difficult to modify.
TeXmacs is as extensible as Emacs, being based on the same principles: having a full-fledged scheme interpreter to modify the behaviour of the UI and to programmatically edit the documents. You can even have Emacs-like keybindings :). What Emacs lacks and TeXmacs has is a sophisticate typesetting engine (i.e. something like TeX) to render complex documents. What Emacs has and TeXmacs lacks is a large user base and dedicated people which develop extensions, but it has a small number of core developers which maintain and use it everyday. It has ~20 years of development history.