Now, I don't know how to develop game with any engine, but when I decide to do it, I will create the game in the same time as I learn how to make it. So research and creation is on the same time. If not, you will lose focus.
This.
Chose a small project and start to make some CG. And when I say "small project" I really mean it. Whatever the game engine you'll chose, make a short (30 minutes long) visual novel with few interactions. It's already a challenge and there's already a lot to learn to achieve this.
Don't care about the quality, don't be too greedy about the pose, facial animation and so on. Just making the scene and a basic rendering is a challenge at start, so stick to this. Then once you've few CG, start coding it.
Each time you're stuck, whatever it's on the CG part or on the coding part, research and learn how to not be stuck anymore. Not only you'll learn more by practice that by pure theoretical study, but you'll also remember it more easily.
The time your project is done, you'll have learned a lot of things but, until you've finished this project, don't go back on what you've already done. You aren't making the game of the year, you are learning the skill you need to make the game of your life, the one you've in mind right now.
Then, you've done it... You just finished your first visual novel, something make by your own hands... Seriously, think about it, think about the achievement it represent...
You are ready, this time create a bigger project, with better CG because you know how to make basic one, you can now learn how to go deeper in quality. Same for the code, no more limitation to a simple visual novel, this time it will be a whole game. A still small game, but a really interactive game. Something like
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by example. It's enough to learn everything you need.
Once again, take the time to savor your achievement, to realize how much you've learned. And when you lack of motivation, think about this, about the pleasure you'll have once you'll finish your project...
This time it's good. You still miss knowledge, because there's always something new to learn. But at least you have enough knowledge to make a full game with good quality CG. Your next game will be even better than this third one, but we all are like this, we all do better the next time.