So VERY random question: if a friend of yours really wanted to get into the KH hype (like ALL the way in [mobile included]) would you go: UX Season 1 ➔ 3 ➔ UX Season 2 ➔ Melody of Memory ➔ Dark Road, or something else? Because I think Season 1 does all it needs to to lead into 3, then 3 gives some questions that Season 2 answers, then Melody of Memory seems to tie in some of MX's backstory, which Dark Road provides.
Because Union χ[Cross] messes with the order of how we experience the climax of χ[chi], splitting the story like that is unfortunately cumbersome.
But when possible I do recommend people experience the story of χ[chi] through to the Dark Footsteps "secret ending," then watch Back Cover, then proceed into Union χ[Cross] proper.
As for where it should line up between the other titles, I dunno. Technically, Uχ launched before KHIII, but of course it's finishing after.
I suppose we could just follow coded's example where it's typically placed after BbS, despite originally launching two years prior.
But again as χ[chi] and Uχ are muddied together now as a single game, the split becomes a little difficult to manage and is reminiscent of people who try to play CoM/Days in chronological order.