Something like LyX or MathType? All I have been able to find is entry via Latex.
I don’t know of any; this was high on the wishlist of the Jupyter creators: WYSIWYG Math editor · Issue #1604 · jupyter/notebook · GitHub but it seems no one volunteered to implement it. There is GitHub - benrbray/prosemirror-math: Schema and plugins for "first-class" math support in ProseMirror! but it is still LaTeX.
But check out this one: GitHub - ianarawjo/notate-jupyter: Tear open drawing canvases inside Jupyter code. In Python, read the canvases as NumPy image data. (probably not practical on most setups but looks cool!).
That is very, very cool, thanks for sharing @krassowski!