> completely impossible to type

It is fairly easy to type if you have configured a Compose key. Usually this can be done system-wide, but Vim for example also supports digraphs, which work similarly. For example Ctrl+K (- gives you ∈, Ctrl+K +Z gives you ∑, Ctrl+K In gives you ∫, Ctrl+K 00 gives you ∞, and so on. You can freely redefine the digraphs to suit your choice of mnemonics. I’m sure Emacs has something similar.

Learning the Compose or digraph key sequences is not much different from learning LaTeX commands. The trade-off is that you can’t infer from the result how it was typed, but in return it is more readable and more compact.