A process with FreeCAD. Import the .stl, select it and use menu part>create shape from mesh, then select the shape and go to menu drawing>project shape. A palette will open with a bunch of options, you can experiment with them to find the result you want and press OK, a 2D projection will be on your screen. You can select it and go to menu drafting>downgrade, this will ungroup the line and arc segments and list them in the labels and attributes palette. Selecting each will highlight them in the drawing window and you can delete the unwanted ones. Then use the menu drafting>dimension, you'll see a circle when you move to a snap point, and dimensions can be done between them. The dimension font size is the second box from the right at the bottom of the header, wheeling it up or down will alter the text to a size you like. Select the drawing objects, then go to menu drafting>drawing and a new tab "drawing viewer" will give you a sheet view, but the scale of the drawing objects will default to 20 and be out of range. Click on the "page" folder in the labels and attributes palette and the drawing object views will list below it. Select them all, then adjust the scale in the "data" tab of the palette to bring them into view at 1:1 size. You'll probably have to move the drawing elements about to get them onto the sheet view, and I don't know any way of doing that except by iteration and creating a drawing, the sheet view tab can be closed off and the "page" folder deleted if it's not right, then recreated when it is. I don't know how to move the dimensions about either, so create them after the placement's right. Once everything's good, menu drawing>export page will ask for a location to create the .svg. I'm still finding out about FreeCAD, but it's got lots of usable features there already. To get projections, you have to copy the objects and translate/rotate the copies to get the projections right, but I can see that with practise, I'd get faster at that, too.
