KNNW is an emotional roller-coaster that finishes in a high pitch. With a (kind of) happy ending; if you have watched Makoto Shinkai's "5 centimeters per second" you'll know what I mean.
But it is a wide open happy ending: you are almost breathless watching >!how the characters climb the stairs... and then.... and then the movie finishes... and...!< what? What happen next? What happens to the characters?
That abrupt ending can easily left the watcher in a state of anguish and happiness... All mixed together.
So, what next?
Nothing. There is no canonical what's next except for the fact that in later 2019 Makoto Shinkai's movie "Tenki no Ko" (Weathering with You) we see >!cameos of Taki and Mitsuha, but from 2021, when they still have not met again at Suga Shrine's stairs (that happened in April 8th, 2022)!<, and in the TnK light novel we learn that >!Taki will get married, because around March 2024 TnK MC Hodaka sees a picture of Taki's weeding at Fumi Tachibana's home (Taki's grandmother). And he married Mitsuha, because while the bride is not described at all, we know she is because !<Makoto Shinkai did confirm that in later interviews.
But how that actually happened? What happened all that time to that point? The short and sad answer is that NOBODY KNOWS FOR SURE. And Shinkai himself has never revealed what he had or has in is mind about it.
Then... what's next?
If you want to fill the gaps, I strongly recommend you to read the canon light novel (or in its defect, its manga) "Your Name. Another Side: Earthbound". It narrates four stories that happen during the movie, and especially before the movie. And it really amazing how it explains and expand the events and, especially, some of the the reasons of the events we saw in the movie.
But still that is not a "what's happen after" answer.
The only (current) answer to that is to reach imagination of thousand of fans. Several of them have draw Fan made comic (like Bleda's "Your name. Thoughts & Dreams"), or have written Fan Fiction story continuations of Your Name, where you can find different possible worlds on how Taki and Mitsuha related or continued their story together. You can find them at sites like FanFiction dot net (FFN) and Archive of Our Own (Ao3) fandoms.
If you want to explore that fan fiction world, I can recommend you the one I personally wrote: "Rediscovering the Future", available at FFN and also Ao3, that I hope can be a satisfactory read that fills and close those gaps of after-the-movie feelings.
Hi OP! I am not aware of any KNNW fanfic that is intentionally canon compliant with WWY. You might need to look for it.
I can recommend the fic I wrote: "Rediscovering the future" (in Ao3; it is also available in FFN). It resumes the story from exactly the final scene of KNNW.
Other very interesting material I strongly suggest you to read is the cannon light novel "Your Name. Another Side: Earthbound". It narrates parallel stories before and during the movie. There is also a manga of the same work, but I'd say the LN is far more better, try reading it.
And in the same venue (filling gaps in the movie story), check u/BledaTG fancomic "Thoughts and Dreams" (beware, contains NSFW scenes, mainly some nudity from Taki not being Taki...).
Other fanfic I've read and I recommend are:
• "Kimi no Na wa: Future" by Alyena2x.
• "Remembering You" by Shining Arrows.
• "Reception" by Brishna.
I suggest "Rediscovering the future" (also available in FFN). It resumes the story from exactly the final scene of "Your Name" movie (Kimi no Na Wa).
Also there are other very good quality fics there. Have you already seen some (you said you've seen a lot)? Depending on that I can suggest you some others...
And, even it if they are not after stories, I strongly suggest you to read the cannon light novel "Your Name. Another Side: Earthbound", that narrates parallel stories before and during the movie. There is also a manga of the same work, but I'd say the LN is far more better, try reading it.
And in the same venue (filling gaps in the movie story), check u/BledaTG fancomic "Thoughts and Dreams" (beware, contains NSFW scenes, mainly some nudity from Taki not being Taki...).
I don't know about your local bookstore, but I found the Amazon listing for Another Side: Earthbound as the very first result on Google.
If you liked the movie, you should check out the light novel. It goes into greater detail of what happened with the secondary characters during the movie's events.