Start with 32 (instead of 3) rock and soil layers in flat creative worlds for easier addition of underground parts like cellars. No more adding soil/rock or poles first to add enough depth for that, unless you want to go even deeper.

As an indicator for how close you are to the void underneath when you dig down, the bottom layer is obsidian and the three layers above it are andesite, while the rest is sandstone and three layers of soil with grass on top.

Layers:
1x grassy medium fertility soil
2x medium fertility soil
25x sandstone rock
3x andesite rock
1x obsidian rock

Instead of:
1x grassy medium fertility soil
1x medium fertility soil
1x claystone rock

If you add the mod after you already started a flat creative world, the thicccer areas will appear in newly generated chunks and - as the generation starts at the bottom layer - will be 29 blocks higher than the rest of the terrain.