Currently it's not possible to make worlds taller than 192 blocks total (-64-128) instead of being able to set it to 384 blocks tall (-64-320) or even be possible to set it to 512 blocks tall (-256-256) whilst also allowing to preserve how sea level is. It would be a great idea to add such things.

It's already doable! By modifying the "height" and "min_y" settings in the modern beta datapack's noise_settings folder, you can have worlds bigger than 512. I've done this for my own world generation, and gotten worlds generating from Y=320 down to Y=-192, for a total of 512 blocks from sky to bedrock :)