I'm sure you've talked about it previously, but can you elaborate on why in your opinion "modular meshes [...] don't let you do level design"?
Here is what Orwell has to say on the matter.
And here are some videos I made many years ago about it all.
https://youtu.be/xOBEy-zIotE
https://youtu.be/6ubu76gEvM8
EDIT: I made another one, god help me. This one is just to demonstrate some basic brush operations/how good Hammer is to somebody who makes tools at Epic Games. It was pretty fruitless in that discussion, but LDs may find it cathartic
https://www.youtube.com/watch?v=U8OgaV6Lu94
EDIT: and here's another video I forgot about that's got some better ways to do things using the third party plugin MeshTool and the UE5 tool CubeGrid.
https://www.youtube.com/watch?v=e7u5tqOCMk8