I’m not exactly experienced in these things, but thought I would share a couple of thoughts.
I started developing with UE4 properly last October (had flirted with it before, but nothing serious), and had to jump straight in and develop a VR prototype in 6 months. It was always going to be a tight turnaround, and not knowing any C++ meant that I decided off the bat to use Blueprints. As is often remarked about Blueprints, it’s amazing how quick it is to prototype stuff and get things working. Without BP, I never could have done it. However, despite my best efforts at following consistent practices, keeping my BPs clearly laid out, etc. by the end of development, it’s a pretty complicated project to dive back into.
For the project I’m currently working on, I decided to take a step back and learn some C++. I’ve also looked at some of the books available, but didn’t really vibe with any of them and, as you said, they are not always up to date.
I started by reading through the entire LearnCPP website, which I found to be pretty clear and relatively easy to follow. I didn’t worry too much about some of the more complicated concepts making sense immediately, I just wanted to get a good sense of the language.
Then I bought the Udemy Unreal C++ course, and I’m actually still working through it. It’s great. The first module doesn’t even go into Unreal, just gets you up and running with C++ first. I found combining that module with reading the LearnCPP website to be a really great way to get started with understanding C++. And from the second module on, it’s in Unreal. I personally think just following that course isn’t quite enough, which is why I’ve been using other sources. I had to read about pointers and references from about five different locations until I totally got it. Might just be me though 
And even though I’m still learning massively, I’ve started on my next project, making it as C++ as possible. Still doing various things (input, for example) in BP, but everything else in C++.
For me, the main advantages are (1) the speed, and (2) the organisation of the code. The speed is only going to be noticeable with certain kinds of things. I’m running some pretty massive loops on tick in VR (90 fps) so speed is essential. It blows my mind how fast C++ is. And although I love the visual layout of Blueprint, I find code in many ways easier to navigate. Both inside a single code file and across the whole codebase. Your question about making a medium-sized game being viable in BP alone is probably more a matter of preference than anything else, especially if you’re a solo developer. If you can keep it all well-organised, and you’re not trying to do anything crazy on tick (which might benefit from C++), then why not? I’m sure there are quite a few medium-sized games that have done exactly that.
The other issue is that you may eventually hit something that you can’t do in BP. I’ve got quite a few things in my current project that are impossible in BP (for example, I want players to be able to import FBX files of their own at runtime, so I had to build that from scratch myself). But BP does a hell of a lot, and Rama’s Victory plugin gives you a lot more functionality. Even if you make your project almost entirely BP though, you might want to at least be aware of how to create a Blueprint Function Library. It’s pretty awesome to be able to write a complicated C++ function and have it accessible as a node in Blueprints. Best of both worlds.
I’ve not tried using the BP nativization to be honest, but I’ve heard that the output is pretty incomprehensible. From what I’ve heard, it’s probably not the best way to learn C++ in UE4. I would recommend get comfortable with C++ as a language first, then understanding UE4’s C++ usage and quirks.
The main thing I recommend, from personal experience, is actually having something to create (nothing too complicated), and be constantly trying to figure it out as you’re learning. I’ve read a few comments from people about the Udemy C++ course where they said they did the course, followed along exactly, and came out the other end having no idea what to do next. If you’re trying to create something on your own, you’re constantly coming up with your own set of questions (how do I actually spawn something? how do I do a line trace? how do timelines work in C++?) for things which you may already know how to do in Blueprints. I find this an important part of the process of learning. Years ago, I decided I wanted to develop iOS apps. I had an 18-hour bus journey lined up, so I bought a book on iOS development with Obj-C and read the entire thing on that bus journey. I understood it all as I read it. The next week, I sat down to start making something and, of course, had no idea what the hell I was doing. I hadn’t really learned anything. It sounds like you’re very experienced in Blueprints, so you’re not starting from scratch, you just need to relearn the C++ way of doing things.
Not sure if any of that helps, but good luck!