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.