How to make a symmetrical drawing where the left side is always (even after editing) a perfect mirror of the right side?
In AutoCAD, you can use Xref for this. First make a drawing consisting of the right half only and save it.
Then make a new empty drawing and attach this right part as a Xref (scale 1:1). Then attach the same part again as a Xref, this time with the X-scale set to -1. Align both halves together.
Any changes in the left or right part (done by RefEdit/doubleclick or by editing the separate DWG) will be reflected in the other part.
Possible texts will be displayed mirrored