I haven't written any epic storygames, so I won't write that article, but I do have some general tips to offer with my experience in starting & finishing long projects. This post doesn't contain any advice on actually writing a story, just how to focus on a project.
1) First make sure you actually want to. Sometimes people want to write epics because they think this is the best way to get a readership & acclaim. Actually the opposite is usually true: people love reading short games, and put off reading the long ones. I think this misconception stems from survivorship bias: as a reader, our favorite games tend to be the long ones that were good enough to hold our attention the whole way; so we assume the length is what made them good. Extra length gives you the room you need to create emotional depth & complexity, but it's not what causes it.
In sum: you should only write an epic if you're in love with a story idea that will actually require that much space to be told. Otherwise you'll be better off sanding down your idea into something manageable for both you & the reader.
2) Then make sure you're actually ready. No one's first project is an epic. Before attempting one, it's a good idea to have already done many smaller projects so you can develop your skills, your comfort with the medium, and your self-knowledge about what interests you. (In your case, it looks like you've got at least a good start with your two games--only you can decide if that's sufficient.) A good technique is to gradually increase your project length & complexity rather than making any big jumps.
Another good thing to look out for is your familiarity with your own writing process. If you can't yet clearly articulate the usual stages your own writing projects go through, it's probably too soon to write an epic.
3) Make sure you have the right idea. Best piece of writing advice I've ever heard was from Kurt Vonnegut: Write on a subject you care about. For an idea to work as an epic, it has to be an idea you care about so strongly that it can effortlessly hold your attention for months (or years!) on end. If you don't care about the idea enough, once the initial passion wears off, you'll completely lose interest.
So how do you find the right idea for yourself? That's a really hard question, but there are some things you can try. Think about what your favorite stories are, and why. Think about the projects you have finished, and what got you through. Think about what first catches your attention with a shiny new idea, and why that shine later wears off. Try writing out a list of commonalities in these areas, and see how many of them you can work into a single story idea. The things you care about most should be the most important part of your story.
I think most (but not all!) people's "key idea" has to do with characters & character arcs. Think about what the most meaningful character arcs you've seen & imagined are, and what the commonalities in them are.
Don't rush the idea. Give it time to settle & mature. Think about it during other parts of daily life, and relate it to other things that interest you. Keep a disorganized notes document and write down every new related idea that occurs to you without overthinking it. Bear in in mind while working on & completing smaller projects. Force yourself to work on it intentionally at least occasionally.
Have multiple ideas like this collecting over time. Be willing to shift focus between them, or combine and split them, and follow your passion. Notice what you're passionate about, and center those ideas. Keep those casual ideas documents alive even while you're working on your main project--that way when you finish or take a break, you'll have something else you can jump right into.
It's tough to say exactly when you're ready to get serious. If one idea maintains consistently high interest for several months that's a very good sign; but you might feel ready much sooner.
The first major part of your project will probably be getting your messy list of ideas in a coherent order. It doesn't have to (and shouldn't!) be a complete outline, just something well-organized. Don't be disheartened if this process is grueling and you feel burned out afterwards--I usually find this is the most difficult part of the project. Take a break to clear out, and then come back to your outline with fresh eyes.
4) Keep a schedule. Any schedule is fine, but it has to be consistent. I recommend picking a daily word count goal so easy you can't possibly fail to achieve it. 100 words is my go-to daily goal. If it's the right idea, sitting down to start for the day will be the hardest part, but once you get going, you'll want to carry on far past that daily minimum. Then again, on some days you really are brain dead and it's better not to force yourself to write more than your minimum.
My writing style is very outline-heavy, so I count outlining words towards my daily goal. Your call if that works for you.
Some people prefer weekly schedules, or ones based on time rather than word count.
5) Don't be afraid to take a break. Sometimes you've exhausted all your ideas on this particular story and you need time off to avoid burning out. The trick is to A) work on something else during your break; preferably a smaller project, and B) Make sure you actually come back once your break is over. Getting distance from your story is a great way to refresh your brain and get a new perspective on it.
6) Once you have a draft, let it sit for at least a month without looking at it. This is hard but really helpful. You need to get distance from the story before you can assess the value of your own ideas. This isn't a post on editing, so I'll stop here.
7) Experiment with your writing process. Try everything once, but don't keep following any rules that aren't helping you. Try outlining sincerely, and ditch it if you hate it. Try a daily writing schedule, and change it up if it flops. This includes all the advice on this list: try it out, and then judge whether or not it was effective for you.
Personally, I love heavy outlining & story structure, and I plot stories from a character-arc-first perspective. Everything else collects around the character arc. I'm not going to go into how-to-outline here since this post is already way too long, but I recommend looking into it. Consider looking into 3 act structure. The trick is to treat structure & outlines as optional tools rather than rules. Use them when they help you and ignore them when they don't. (If you end up hating outlines & structures, that will be "always".)
8) Follow your passion. I've already said this, but it's honestly the most important part. Make the things you care about the most important part of the story. Take the story in the direction you want to see it go. When deciding what to do on a day-to-day basis, work on the part of the story that you're currently most excited about. When you do need to grind out a task you're less interested in, see if you can change the story or task in some way so that it interests you more. If you're bored writing it, the audience will be bored reading it. Having fun with the writing isn't a guarantee of quality, but I do think it's usually a prerequisite.