You should play in the order they released, not in the chronolical one so:
Kingdom Hearts 1
Kingdom Hearts Chains of Memories
Kingdom Hearts 2
Kingdom Hearts Days
Kingdom Hearts Birth By Sleep
Kingdom Hearts Coded
Kingdom Hearts Dream Drop Distance
then in either order:
Fragmentary Passage 0.2
Kingdom Hearts Unchained
My (locked) thread about how difficulty should be a decision for the developers, not the gamers.
https://gamrconnect.vgchartz.com/thread.php?id=241866&page=1