no offense at all intended to the person who made the post in question but I disagree with their recommendations for play order and wanted to offer a counterpoint (should this be a rebug? idk. this way felt less rude)
and I am really not trying to be a gatekeeping asshole when I say this, I swear! but speaking from my own experience, there's not really that much wiggle room in the play order if you want to have a good time. at different points in my kingdom hearts gamer life I have skipped a few games here and there and I always, without exception, have a million times more fun after going back and playing in release order(ish)
as a kid, I skipped chain of memories. I liked KH2 a lot, but it was so much better when I replayed it years later after playing chain of memories. I legitimately hated KH3 and dropped it very early on when I played it originally because I was so lost, and that was from skipping two games! I was so confused and frustrated that I decided the game was bad and I didn't touch the series for a couple years after that. but when I came back to it and filled in those gaps, it felt like a completely different game! I love KH3! it's amazing! besides all the things about it that suck!
so, again, this is just me trying to encourage people to set yourself up for a good time. I agree 100% with the OP that kingdom hearts' reputation for being incomprehensibly convoluted is very much overblown. there's just a lot of story there, and it's all connected, so it's very easy to get lost if you're missing pieces of it. with all of that being said, here is my recommended play order (and I won't be getting into which version of the games to play, how to play them, etc. the OP has good info on that stuff)
- kingdom hearts 1. absolutely no reason not to start at the beginning imo. it's a pretty self-contained story and it's the foundation of everything that comes next. great game
- chain of memories. this game establishes so many characters and mechanics that will be important in every game for the rest of the series. excellent character arcs too. great game
- kingdom hearts 2. many will tell you this is the best game in the series. they're right! why not play it next? great game
- 358/2 days. probably fine to play before KH2 (it does take place first chronologically), but if you want to get a little silly, I recommend playing up to day 6 of the KH2 prologue and then pausing to insert this game into the middle. the thing about these two games is, whichever you play first, you're spoiling some things about the other one. personally I think the things that 358/2 days spoils about KH2 are easy enough to suss out if you're paying attention, and the character arc is just so good this way. you get to enjoy the mystery and confusion of the prologue of KH2, and feel the impact of the story that led to it. good game
- birth by sleep. this one, being a prequel to all the others, arguably can slot in just about anywhere, but imo there are some mysteries seeded in KH2 and 358/2 days that have much more satisfying payoffs if you play this after those. great game
- coded. I wish I could say this was skippable, because frankly 90% of the plot is completely inconsequential. but it introduces some inexplicably very important concepts and that last 10% is bizarrely good. this can probably go before birth by sleep, but it doesn't really matter. game
- dream drop distance. the OP describes this game as a bottleneck, and it really is. everything that came before it comes together here, and everything that comes after it is building on this. I honestly don't think this can be played at any other point besides release order. seventh or bust. great game
- X / union X. a gacha game way-way-way-prequel that has some fascinating connections to the present day of the series. so much better than it has any right to be. I highly recommend following the instructions of this cutscene compilation from damo279 - watch everything that was released prior to fragmentary passage, and come back for the rest after KH3. great cutscenes (haven't played it lol)
- fragmentary passage. a short prequel to KH1. again, this builds on stuff from dream drop distance and sets up stuff for KH3. play it here! great game
- kingdom hearts 3. again, speaking from experience, absolutely one million percent play this after everything above. there is so much stuff that will make no sense whatsoever otherwise. it's the conclusion of a massive story arc and the extent to which it ties up plot threads with a nice little bow is ludicrous. so so so so much more impactful when you have all the pieces. great game
- the rest of union X
order doesn't really matter for the next two - dark road. again, just look up a cutscene compilation. like coded, a lot of the stuff that happens in this is kind of pointless, but man when the plot kicks in... WHEW. great cutscenes
- melody of memory. if you want. this game is immensely skippable if you aren't interested in playing a decent but not $60 good rhythm game. there's like half an hour of cutscenes at the very end that you can watch on youtube. okay game
so there it is! that's my take. "take" it or leave it (haha.) if you are a person who has already played some of the games out of order, I highly recommend starting at the beginning and going through for the whole story. it's good! and imo it's only gotten more interesting over time. happy gameing