hi, i have worked on many unity 3d projects for mobile and pc also i have experience working with unity's shader graph , and i can work on this project, but am not sure i fully understand you, what is STL scene?
however for the part I understood, which is building realistic scene with navigation controls, ability to highlight objects control them and move them with the collision detection, and the save and load functionality, and the multiple users logins to the same scene( also multiple users control the objects), and the simple UI to select tools, i can build you demo for free of project that do these things already, and i will send you video only of what i built, if it was what you needed then you can select me for the project, other wise it just for free.
please massage me anytime so i can understand more, then i start working on the demo.