As a BNG fan, the modding capabilities inspired this project:
https://youtu.be/acW5RGl6HGg

As the description says, it is a model creator, not a "level editor".
it can ouput .trm files that are used in beginning of the scene authiring process
It should be available (freeware) when completed ans stable enough
(I'll try at the end of the year, after the BNG release...)