Some additional points of interest: Quake level designers could use "hint" brushes to help the BSP compiler determine where to create cells. Starting with Quake II, designers were able to place "area portals" which are portals programmers could toggle at runtime (think disabling a doorway portal when the door is closed).
I remember discovering all these optimizations The Hard Way when making maps for id tech 3 games as a kid. I always tried to build grand and detailed spaces, and the BSP/vis would choke on all the geometry.
> What doom mapmaking needs is higher level tools. Like a tunnel kit. Just plug together pipes and fittings. Or crank out mazes generatively.
You're describing the contemporary Unreal Engine, which was "solid by default, carve out the space" kind of a thing. GtkRadiant (the id tech 3/4 editor) had all the basic CSG operations built in, so you could theoretically carve out spaces, but Radiant's CSG was widely regarded as "evil" - I never found out why.
