I’m in a very similar situation, or rather I have been for a couple of years now. I’m working on a personal project that’s way too big for a single dev and may or may not ever be released. But I’m still chipping away at it whenever I find the time after work. In my opinion, the hardest part with these personal projects is to maintain your motivation to keep going. It may sound silly to you right now, but I know tons of people who gave up on their games or hop from one unfinished thing to the next because they can’t keep that initial passion for it going. As long as you still have this passion, it’s just a matter of time until you get the results you want.
When you’re starting out making games, you’ll often hear the advice to start small. However that doesn’t mean that you have to create tiny little games you don’t care about. Simply break down your project into smaller individual chunks that you can manage with your current skill set but still contribute to what you’re ultimately trying to achieve. Don’t start by creating a customizable character that can attack enemies with upgradable magic and self-crafted swords and block attacks on a physics-based system with tons of cool animations. Instead, start by using the default UE4 assets to create a character that can simply move around. Then give him a weapon and try to make him hit stuff. Then try replacing the UE4 assets with your own. And so on and so forth. 
Creating the art for your project can be a tricky subject. You have to start working on it to get better. But as you get better, your old stuff will look bad in comparison. And before you know it, you’re in an endless cycle of recreating your old assets to bring them up to speed with your latest work. That’s why I would recommend setting a high (but realistic) standard for your project and only start creating assets when you’re good enough to match it. Until you reach that point, you can use the free assets that come in the various UE4 sample projects.
You may also decide that creating all the art yourself is not an option and that you’re better off buying assets from the marketplace, but I would advise against doing that until you’re well on your way and know exactly what you want to create. Buying gameplay systems can be useful early on for taking them apart and learning from them, but for basic stuff you’ll also find free tutorials or sample projects.
As for your actual project idea, I would recommend looking into the existing Sword Art Online games and maybe MMOs and compare them with your personal vision. Those games probably won’t hold up to what you want to create, but try to identify why they don’t hold up and how it could be fixed. Is it even possible to achieve what you have in mind with a video game? What is the core of the experience you want to create? Try to distill what’s special about your idea, which part of the SAO you want to capture, and build your project around that. As you start creating your game, you’ll get a better understanding for what you can accomplish and what’s out of scope. Knowing what the essence of your game is will help to cut only those features that are not essential to this essence.
I can’t give you much advice on studies, but based on my own experience I can echo the sentiment that most “game design” degrees are not worth your time and money. I managed to get into the industry despite the almost worthless education I got from such a degree, but that’s only because I spend all my free time learning this stuff on my own.
So yeah, hopefully you can get some useful information from all this rambling. If there’s anything more specific you want to know, feel free to ask me here or via PM. Keep in mind though that all of this is just my personal preference and experience. Take from it whatever feels right to you - The most important part is that you can keep the fun and passion for you project alive. Everything else will come naturally!