I wonder if the author considered an input method where keypresses translate directly into a rendered equation? Symbolab kind of does this, so instead of writing an expression in some functional style and seeing its rendered output, I can directly manipulate the rendered equation with the keyboard.
The other point the author makes about discoverability of operators like sqrt and cbrt, I feel that you could circumvent it by introducing a semantic search feature for operators. For example, typing "square root" would display a list of suggestions for operators that match that, like an IDE.
The last question I have is if the author has looked at libraries like Calcium (https://fredrikj.net/calcium/) or the work that went into the Android calculator recently (https://dl.acm.org/doi/pdf/10.1145/3385412.3386037). They use exact arithmetic with computable real/complex numbers. It might serve as a cleaner abstraction that switching between different BigNum representations yourself.