You can alway write your own Leveleditor, that uses the ingame rendering engine.
Because then you have the tools that fit to your style of game.

Exerything in a 3dgs Level, except the "BSP Blocks" could be create with our own editor.

If you are really working on an ambitious project.
Using your own leveleditor, or at least a "level refinement" editor
is the way to go.