First attempt, the frame.
Reminder of the construction flow :
After integrating the dimensioned images of the project at scale 1 and in order to limit calculation times when the body has several Sweeps, I chose to build the frame in several bodies joined together at the end of the construction site by a Boolean union (operation included in PartDesign LinkBranch).
The principle of construction is of course the same as that of @ppemawm, except that LinkBranch makes it possible to remain in PartDesign.
I also incorporated solid bars: the problem of tube joints can be circumvented by an internal sweep of the main tube at the end of the run, but the files quickly become huge.
(https://forum.freecadweb.org/viewtopic.php?f=24&t=69101 )
I noticed that after a 3D sketch sweep, the result is not automatically merged with the previous step, although the Refine property is True. This required making multiple steps visible simultaneously, which is usually not possible but oddly sometimes worked.
To overcome this problem I decided to perform intermediate Boolean fusions. It seems abnormal to me.
Following a comment from @ppemawm, his file is 13.1 Mb while mine (full) is 212 Mb !!!
I want to believe that the fusions that I have regularly added are responsible for this, but I would like other opinions and remarks.
The file : https://www.dropbox.com/s/okx8ym5r0t1tc ... FCStd?dl=0