I personally don't attach whatever KH3's flaws are--and people disagree on what those are--to the fact that other games were in development. I usually put it down more to Nomura not really caring about the KH series anymore as much as other games he works on, with a great helping of bitterness over FFXV thrown in. A lot of things about KH3 just felt like he was over it and decided instead to use the time working on KH3 to morph the KH characters/story into something that he didn't get to complete on FFXV which will take full direction of KH post-3 (to KH's detriment, imo).
I'm gonna be honest, and maybe this is a hot take here (none of this is an attack on you, but moreso a general statement, now that the opportunity is here) but I HIGHLY doubt he doesn't care about the KH series anymore, I feel like this is the biggest overreaction ever, and people need to chill.
Okay, so KH3 isn't the masterpiece people were hoping for. Disappointments happen, it is what it is. But just because certain things don't come out the way you hope, doesn't mean the guy absolutely doesn't care about the series anymore. He clearly does care, because he still wrote the story for KHUX, he's still doing interviews for KH ReMIND, he's still appearing at concerts... All of this on top of everything he's doing for the Remake. It's the story he wanted to tell, and like it or not (again, not directed specifically to you), that's what we're getting moving forward.
If he didn't care about the series as much as he used to, he would've never introduced Back Cover, made KHUX canon, and put the Foretellers in the epilogue. He would've just wrapped the whole series up in KH3 and not cared if there were unanswered questions or the game felt too congested. But that isn't the case. In fact, KHUX is looking to be the vehicle for new KH story from here on until whenever it ends OR the next game is announced. So it's clear he does. Maybe he's overeager, sure, but the guy still loves the franchise a ton.
And addressing the FFXV stuff too... What if it was always meant to be combined? Nomura mentioned that Sora and Noctis were named for specific reasons, specifically in terms of how their names are opposites, so who's to say he wasn't playing the long game and Noctis was going to be in the KH-Universe to this degree anyway? We'll never really know for sure unless he comes out and says it. And he can't (at least right now) because of just how taboo Versus XIII as a topic apparently was in the company. But seeing as how much groundwork this would take, it probably was intentional even back then.
Speaking more personally, I'm a creative. And from my perspective, if I had an idea taken from me and not even have it done properly according to my own vision, I'd feel some type of way too. People write it off as "lol Nomura stop being salty" but this clearly meant a lot to him... And the same people who say that, are the same ones that get super triggered when their art is posted on a forum or Reddit without permission or stolen from their Patreon (which is justified, btw). It's easy to say "get over it" when it's not them who has to deal with it. I get why some people wouldn't be too keen on it, but, it's still his creation.
I think... It's getting to a point where some people may just have to come to grips with the fact that where the series is going, it might just not be for them anymore. It happens. You love things for a time, and then your taste changes, and suddenly it's not your thing. But everyone's opinion is valid. I'm not telling anyone to leave the franchise or fandom, but I do think there's gonna come a time when people will need to ask themselves if they still want to be on the train. And if not, that's fine, you still have all the games that stuck a chord with you. As for me, I'm still more than excited to be here, but if the stuff is turning you off... It might just be time to step away for a while, and there's no shame in that.