Can I ask for how to remedy my build here?

I learned this technique from a video, I don't remember who the author was.

Their idea is to start by building hollow cubes of somewhat random sizes, each connecting to the other and possibly merging (some parts of a cube inside others). Once you have enough cubes, you use those as the skeleton to build the final structure. Square-ish cubes become rooms, tall and skinny cubes can become stairs, really big cubes can be multi-floor open areas. The hollow cubes act as a canvas with structure, so that you can "paint" your build on something more than just empty air.