You need a none of the above option. Because none of those apply.
Your problem is that your typical story is about 100k for a reason +/- 20k or you can go the Infinite Jest/War And Peace option and make a 500k monster, which can work when the story demands it. The problem is that both are also a product of their time and really don't translate well into modern age.
Your problem isn't that your story needs to be 100k, but that your story need to WRAP UP at 100k.
Heres the mechanics of it.
Book 1 - Act 1 Act 2 Act 3
Now you need a plot arc for each act, then a plot from act 1 to 2, from 2 to 3, from 1 to 3.
That's 6 plot arcs.
Now let us assume we're doing 120k a book, 40k an act, say 4k a chapter 10 chapters an Act
Each Chapter needs to in some way add to a plot in some way, so you have: 3 plots in act 1, 4 in act 2 and 3 plots in act 3
Now act 1 needs to do some heavy lifting, because it's got your intro and act 3 has your post climax So in reality it's 4/4/4 that's 10k words to devote to each "plot arc" for each act, or 2.5 chapters to devote to each individual plot arc/intro/epilogue where it needs attention.
4 Seems to be the magic number for what a reader can handle. Don't ask me why, it's just what they seem to be able to juggle without getting pissed off.
Now you can add more to that, but when you get to 70 chapters, (lets say your chapters are only 2k words) you are looking at the point where you have simply overstayed your welcome.
Either you are putting too much fluff to bore your reader, or you are adding in too much new shit to follow and turning them off because they can't keep track of everything. Now, that isn't to say you can't add in foreshadowing or plot hooks to direct the reader to the NEXT book, but that shit needs to be kept to a minimum.
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Let's look at a trilogy. We add:
Book 2 - Act 1 Act 2 Act 3
Book 3 - Act 1 Act 2 Act 3
Now, you need a plot act for Each book, adding up to 6 per book, or 18 total, just for basic bitch work. But if you are making a proper trilogy, each book needs to flow into the next one to some extent. So you also have, the potential for plot arc from:
Book 1/Act 1 to Book 1 Act 2
Book 1/Act 1 to Book 1 Act 3
Book 1/Act 1 to Book 1 Act 1
Book 1/Act 1 to Book 1 Act 2
etc etc etc
Book 1/Act 1 to Book 3 Act3
That's 9 plots starting in the first book, first act, not including the introduction, so we have 10 things to juggle Then we got 8,7,6,5,4,3,2,1 added for the next 8 acts, with Book 2/act 2 potentially having to juggle 8 plots from act 1 moving through/ending in book 2/act 2, and 8 fresh ones beginning/ending in book 2/act 2. or 16 plot arcs to juggle.
Do you see why most books stick with the classic 3 act structure and are basically encapsulated?
Just adding a fourth act makes for 4,3,2,1 or 10 plots with 5, 6, 6, 4 plots to handle, including intro and epilogue for a true 6,6,6,5.
Now if we stick with 10k words devoted to advancing each plot arc properly, that's 60+60+60+50 or 230k words needed to properly flesh out a four act structure. Now, you can minimize it, combine shit, you can bring that down 130k, if you are really lean and mean when it comes to writing and are good at having shit pull double duty with scenes properly advancing multiple plots at the same time, but you GOT TO BE GOOD.
A true trilogy, done with a proper 3 act structure per book is just a fucking nightmare.
And the truth is, you might say, "Well, I don't need to follow you stupid plot structure! I don't need X plots per act! I don't need them to arc between acts! I can do things my way!"
Yeah. You can. But it doesn't sell. Editors have these formulas for a reason. The industry has been doing this shit a lot longer than you have and I promise you, they are much better at making money than you are. Half the work I do is just making filler plots. I don't do the main story, I do the B and C plot. They stick to this formula because it works.
This is why "Books" are 80k to 120k.
This is why your story starts to die at chapter 70.
It's not your chapter limit, or your word limit, it's your ability to juggle plots without pissing off your reader by being too complicated or being too simple.
Now, you can do a plot structure with Episode 1 to Episode 2 to Episode 3 to Episode 4 to Episode 5 to Episode 6 to Episode 7 which is when you get your combination Bottle episode, "season arc" model. Where each Episode is 1 plot, but there is a "Season Arc" so that each "Episode" is basically a single episode of a TV series. Understand that then each "Episode" is back to our 3 act structure, except that it is One plot from Act 1 to 3 with maybe a B plot (that should be brought back in to support the A plot) and an "over arcing" season arc that is sprinkled in. Then you can stretch that shit out fairly far, but you have to be careful to do 90% bottle episodes, with a few "2 part" episodes here and there, but it's best not to do that too much. Maybe at half way through the season and at the end.
That... doesn't lend itself to writing, however, but if your goal is to push it into another media, hey, start off shooting for what you want. Every script has to start somewhere, right?
However, the bottle episode/season arc writing method can, theoretically, go on forever. So, if that's what you want, super long series of writing, then go this method. made your stories like: Intro, Act 1 to 3 arc, epilogue, with a season arc thrown in the middle. So you have for act 1/2/3 you got 2/2/2 With act 1 being the bottle plot and intro, act 2 being the bottle plot with season arc, and act three being bottle plot and epilogue, or about 60k words, which you can reduce to down to 30k with a lot of work.
If you give the story in 30k to 60k Bottle Episodes and every 20-odd episodes, wrap up the season arc, you basically are writing a 1/2 years worth of 'comic books' or a TV show series. and you'll avoid reader fatigue.
You have to let the reader BREATHE. You need to give him a place to STOP. You notice how I break up my paragraphs in a very odd way? How the line breaks are all over the place? Some paragraphs huge and others are one line?
That's to drive home the point of this.
And to also give your mind a signal that here, you can stop and look away and think about what I typed.
You need to write the same way. I suspect, without reading your work, the problem is that you aren't giving your readers a break. They don't have a place to rest. This leads to plot fatigue and thus the drop off on view count.