I work with several software packages (AutoCAD/Rhino/Sketchup/Blender) that all have "Mirror along line" capabilities or use plugins that do. I live by it.
Though I haven't been using Corel as much as I used to, I'm currently back using it. (2018) I seem to remember at some point I had a script or plugin that gave me mirror on line capabilities. Perhaps CADTools 5? Or maybe it was a how to video. Any suggestions?
There is this freebie one that still works in CD2018: http://www.corelvba.com/index.php?pages=mirror_1
There is also a Mirror feature in their CadTools5, but I believe it works the same. Can't check -- got a new computer and cannot justify the cost yet.