Oh man I wish I had this when I first started!! Amazing resource! I figured I’ll chime in with what I know:
KH All-In-One does not include Re:Mind, the DLC expansion for KH3. You’ll need to buy this separately, or watch a cutscene compilation online.
What’s going on with χ: 2.5 ReMix Edition
KHχ was originally a browser game, only available in Japan.
KHuχ is the mobile game currently available, and covers most of the KHχ story, however it is not a direct port and is missing the final few chapters of KHχ for plot reasons.
Union Cross is something of a second season to KHuχ, and continues where the story left off.
KHuχ is currently ongoing, but is nearing the end of its life-cycle. I wouldn’t write it off as plot-irrelevant - there’s some set-up for the future of the series.
That said, I absolutely would not consider it necessary for understanding up through KH3 & Re:Mind. How relevant it will be is yet to be seen.
KH Dark Road is a recently released (June 2020) mobile game which is contained within the KHuχ app - you will not find it in an app store.
The game is set to delve into the backstory of the antagonist Xehanort, but is appearing to be heavily linked with the KHuχ plot.
Both KHuχ and Dark Road are renown for being incredibly grindy, and do have gacha elements. If that’s not in the cards for you, but you’re still looking to be caught up on the latest KH developments, Everglow on Youtube has English-text uploads for each chapter, including KHχ and Dark Road (as it comes out.)
KHχ (English Translation)
KHuχ (Including Union Cross. Covers current NA releases and translated versions of JP updates. Keyblade War event included at the end of the playlist.)
I think that’s it! KH is long and the Disney stuff can be off-putting, but it’s a lovely, genuine series once you jump in. I can’t recommend it enough.