The LyX editor for math is ridiculously competent. Even using plain LaTeX, LyX can be used just for the maths because it is so quick. I can't imagine the math for which LyX would be insufficient.

It doesn't render like tex in the editor, but it is logically correct.

LyX has a "tag" for plain tex, which allows you to insert any tex file into your document. That is a practical way of working with TikZ, for example.