I’m seconding what others have said about “fluency” being something that is hard to pin down. You’ll find yourself more or less “fluent” depending on the genre you’re reading, and how much prior experience you have with it.
Like, just to give an example, I’m still an upper beginner in terms of overall Japanese level, but the vast majority of my immersion has been Japanese pro wrestling, so I can open a book about Japanese wrestling and actually understand a fair amount of it just due to the fact that I’ve specifically studied the vocab (I have 840 pro wrestling words in Anki), and I know many names of wrestlers and many details about matches and stories from a variety of different companies, which helps provide a lot of context when reading about it.
I have books that are, well, not about wrestling, and the difference in fluency I experience is truly incredible. When I open one of those, I have absolutely no hope without a dictionary. Skimming gets me maybe 20% of what’s going on, whereas with some wrestling stuff I can skim and understand easily 70-80% without needing to look up any vocab or kanji.
If I keep working at this for another year or two, my fluency with wrestling stuff will just keep climbing higher and higher. And if I approach another genre with the same dedication that I’m applying to wrestling now, I’ll improve my fluency in that area as well.
Of course, none of these things exist in a vacuum, so you’ll get benefits in other areas from focusing just on one genre. Pro wrestling has, believe it or not, given me vocabulary that has helped me understand senryu poems.
As far as how this dovetails with progress through WK, I think that’s hard to say. Most of my gains in reading fluency have come from applying my WK knowledge outside of WK. I do use my WK level to sort of gate the vocab I add to Anki, though. I only add words if I already know the kanji (or if the kanji isn’t in WK, I learn the kanji on my own and then add the words to my vocab deck).
I didn’t start actively mining words from native media until I started translating wrestling promos and recaps at the very end of last year. At the time, I was between N5 and N4 in grammar level and almost halfway through WK, so the vast majority of kanji I encountered, I recognized. There were no shortage of new words for me to learn.
Now, with wrestling, I add new words at a much, much slower rate, because I’ve learned quite a bit of common vocabulary already. When that rate slows to a point where I’m learning only one or two new words each wrestling translation I do, I’ll probably start branching out and SRS-ing words from other genres.