It might be simpler to create an "other"-handed configuration of your sub-assembly.
Just wanted to point out for others at least since this a bit late.
If you select one part in the assembly (1.) and hit the Mirror button. The Mirror sign comes on that one and the assembly (only those two), then you only need to deselect it from that only part. That leaves the assembly mirrored and nothing else.
Saves a few clicks