Is the investment in Webpack worth my while?
Maybe. Depends if you are going to use it. If CRA's default is all you need right now, no need to complicate. I would say its worth while having a play with it, at least to get as far as knowing what it is doing for you.
Is it likely to be phased out soon or is it still essential?
Depends what you mean by "soon", by JS standards, its probably going to be around for a while. It seems somewhat settled as the defacto bundler / build tool at least for the time being.
For my next project I want to get a bit more complicated and use react with express. Will Webpack be helpful here?
Maybe, but probably not for the time being. Best to just follow the electron / node way of doing things without looking into how you can use webpack on that side of things also.
can I progress with create React App
Sure. There's no reason why not. CRA is using webpack to build your app. It comes with a config built in, and a fairly complex one I think. Its entire purpose is so that you can skip all the set up and just get started. If you're still learning react, its maybe best to just focus on that.
From my research Webpack is fairly tough to learn.
I wouldn't say its hard to learn, its just there are a lot of options, and a lot of things it can do. So, to know what all those things are, and if / why you want them takes a fair bit of reading / experimentation / experience. The config files are all in JS, and the documentation is good, and there is plenty of examples out there.
I you do decide to look into it I would suggest something along the lines of:
``` // entry.js import second from './second.js'; console.log('I am entry');
second(); ```
``` // second.js
function second() { console.log('I am second'); } ```