dimensions and biomes

To change the height of custom biomes, edit the "erosion" value in the generation tab of your biome element. Higher values result in flatter terrain. Lower values result in jagged, high terrain.

Regarding changing the stone block that generates below y-0, and the bedrock, you need to edit the Java code by right clicking your dimension element and clicking "Open mod element in code editor."
Then click the dropdown option ending in "mod_name\dimension\dimension_name.json"

A text editor opens up, here you can use Ctrl+F to find the specific text to change, or just scroll.
To swap bedrock out for another block, look for the bit that says the following:

{
            "type": "minecraft:condition",
            "if_true": {
              "type": "minecraft:vertical_gradient",
              "random_name": "minecraft:bedrock_floor",
              "true_at_and_below": {
                "above_bottom": 0
              },
              "false_at_and_above": {
                "above_bottom": 5
              }
            },
            "then_run": {
              "type": "minecraft:block",
              "result_state": {
                "Name": "minecraft:bedrock"
              }
            }
          }

And change the "Name" property from "minecraft:bedrock" to the registry name of whatever block you want.

If I recall correctly, MCreator dimensions don't contain the code needed for the generation of deepslate-esque blocks.
You may be able to find code online. In that case, it is a similar process; swap out "minecraft:deepslate" with the block you want.

Oh yeah I think you may need to lock the mod element after editing it's source code. This makes it difficult to edit any properties or add any biomes afterwards, and doing so would reset your edited code.
Locked code also cannot be regenerated and may break with Minecraft or MCreator updates.
 

Be warned.