Currently, bedrock edition already have capability of flat world layers customization, but this option is hidden! Please tell why it's hidden!

If developers will not add GUI for in-game creation of customized/deeper flat worlds in bedrock edition, then flat world option should be removed from world creation menu! Creative building on four-layer flat world is EXTREMELY BORING and you CANNOT BUILD UNDERGROUND STRUCTURES. There are external site https://mcbe-essentials.github.io/world-generator which can be used to create customized or deeper flat worlds for bedrock edition!

If you open bedrock edition world with NBT editor, you may see FlatWorldLayers NBT!

There is a https://mcbe-essentials.github.io/world-generator/ site that can create bedrock edition worlds with hidden customization options!

In order to import .mcworld file into MCBE on some platforms (Android), You currently may need to use different than default file manager (You can use Total commander, ZArchiver and else)