Currently, custom dimensions are just copies of existing dimensions. You can change a bit using the json, but nothing much significant. If possible, more in-depth configurations for things such as skylight, mob spawning, fog, even skyboxes, would be helpful. I'm unsure of how much of this is actually possible or even within the scope of the mod, though.

The skylight and mob spawning is determined by dimension type. You can add new dimension types via datapack.

The fog is determined by biome. You can add new biomes via datapack.

About skybox, as far as I know vanilla does not allow customizing skybox via datapacks, but you can search for other mods that customizes skybox.