I have read a couple 19th century books about fictional heaven which were more successful than this, but they started with the main character's death and spent the whole book exploring heaven, and the whole "exploring an interesting new world" aspect makes it feel more like a portal fantasy.

George MacDonald's Lilith (1895) has the shtick of finding true life in dying, but it also has such a personal, unfiltered, gonzo vision of Christian myth mixed up with midrash that I don't feel insulted by it, it's exactly the kind of flight of id-fueled fancy that the ending of The Last Battle closes down.