if I Mirror - I have to constrain every (mirrored) part again!
Can you take a screenshot and post here what you are trying to do? I may have had this same experience a while back and I have an idea of what you should do.
See PNG. attached.
If I mirror the left-hand rails to the opposite side.
I have to do all the constraints again - so there is no point in mirror, take the same time.
I see what you are trying to do. The easiest way to do this is to use the "create component" command. You'll right click in your assembly, and click create component. It will have you save your whole assembly as an ipt file. Once that creates the ipt file, open that file and then mirror that part on a plane. You will then make sure delete original is selected when mirroring in the ipt file. Then you can place that part file in your assembly and constrain it as needed. Yes there is still constraining and new file creation involved, but it will save you so much time.