I don't think Unreality is the afterlife for people in the Realm of Light.
It's a "world of death" to Sora and Strelitzia because they "died" in a sense before they went there, and so is a thematic afterlife for them, but it's not "the afterlife", period. Neither is there because they "died and went there"; Sora vanished from reality itself while Strelitzia was killed normally but was seemingly recreated from her data, and was seemingly sent to Quadratum from there (something like that).
Conversely, Stella (or whatever her new name is) is an inhabitant of Quadratum whose heart went to The Final World when she died. Maybe there's a plot reason for this, but perhaps the Final World is the afterlife for people from both sides of reality (Riku had to go there to open the portal to Quadratum, after all).