Please Help - Dimensions and Biomes

Biomes 1: Go to the generation options for the biome, each question mark will have info about what values apply. Make sure to have your mountain biome far inland whereas your plains biome would be near the coast and inland. For the mountains you would want high erosion and high weirdness with the lower half set towards zero for each.

Biome 2: A 100% flat biome isn't possible, but you could have a biome with extremely low weirdness (example: -1 to -0.9) which would make the biome spawn only in nearly flat terrain, just adjust the rest of the parameters for world placement. Keep in mind that having such a low range for weirdness will make the biome pretty rare before you go messing with anything else.

Dimension 1: Make the dimension however you'd like. Create a feature that spawns in every biome in that dimension. Use the dirt blob template and have it set to replace the list of blocks containing deepslate. Set the number of tries extremely high, there is a question mark box that recommends maximum sizes, max out everything. This will replace a large majority of deepslate. Unless anyone else is aware of a better way, that's what i've usually done. Custom bedrock to replace bedrock isn't possible.

Dimension 2: When you make a dimension there is a box to add an igniter texture. Add your own igniter texture. It will make the igniter as an item. You can then create a procedure with that item to do whatever you want.