Years ago I looked into both LyX and TeXmacs. I could already quickly write math in LaTeX but I wanted to work out calculations on a single screen without the code/renderer split screen setup (having the math directly visible is a game-changer for doing long calculations, by the way). TeXmacs won hands down for me. I had no problem learning the intuitive shortcuts (e.g. any letter - TAB for its Greek counterpart). If you know the LaTeX counterpart you can almost always type that in. The converter to LaTeX source is quite good. I love that the "nesting hierarchy" in a math expression is visible. For instance,

"(x^2 + 4)"

has four distinct components: the parentheses "(...)", the term "x^2" (two components, "x" and "2"), and the term "4". If you place your cursor before or after the parentheses and use SHIFT+RARROW or SHIFT+LARROW (respectively), you highlight the parentheses; if you do the same before or after the x^2 term, you highlight x^2. I find a lot of math editors (e.g., Word 2016) get this wrong because they will just highlight the parentheses or the exponent. Smart highlighting is super useful for fast manipulation of equations. Oh, and also -- if you are within a set of parentheses, you can keep hitting TAB to cycle through (), [], {}, ... . Not to mention, TeXmacs handles all the \left and \right parentheses sizing for you (the parentheses in the above example would expand if, for instance, you made 4 a fraction like 4/3 -- a quick Meta-f away)