Following the datapack instructions at https://terrafirmacraft.github.io/Documentation/1.18.x/worldgen/dimension
I created this file: https://github.com/Gaelmare/TFCGyresRockLayerSize/blob/master/src/data/minecraft/dimension/overworld.json

When loaded into a new instance and generating two worlds, with and without this datapack file, no changes to rock layer size appeared via Blazemap/Cartography nor from actual inspection of the rock layers in world.

Attached is the vanilla world generation settings export, showing the rock layer size changed.
worldgen_settings_export.json.txt

Attached is a datapack which works and shows that yes, this is functional, suggesting the issue is not with TFC but is between the keyboard and chair.

Datapack in question: tfc-rock-layer-size.zip

Thank you for testing. PEBKAC indeed apparently.

TIL that if you "Re-Create" a world that used a custom datapack, even if that datapack is no longer shown in the datapack list, like you launched Minecraft without the mod containing that datapack, that custom datapack's settings will still apply.

Wack.