It looks like 3 separate pieces(combiner, mirror and laser holder) and I can’t see how they are held into the 3D printed mount. I just figured it was hollow or something so that screws from beneath could hold things in place. That’s why I asked about posting the files so I could look at either the FreeCAD design or the STL files and see how you did that.
As for FreeCAD parameterization with Spreadsheet, it’s easy. click the spreadsheet workbench, create the sheet then I use 2 cells for each parameter. A is for names I can read, B is for the value. Then you right click on the item in columb B and select alias and type in the label you want to use in your design.
example: Laser holder OD , 5.0
alias is laserOD
Then where you’d type a number in FreeCAD GUI you can see a little eyeball or thing to the right in the input field. When you click that you start typing Spreadsheet.laserOD/2 for a radius entry.
I would not worry about changing yoru design now, if someone knows FreeCAD they can easily update the design as needed. And your K40 looks alot like my K40.