What I do is save a copy of the project, and then rename the bones and actions for the mirror side. For example, if your bone/action has an R at the end, change it to an L. When renaming actions, you can select the root group of the item (i.e, the 'arm' group,) and rename the actions for that. The renaming will propagate to the items inside the group. Note: select ONLY the group...this automatic renaming won't work if you also select the layers inside. Also, you MUST do this in a separate project to avoid naming conflicts.
Now, with the original project open, use Copy Layer and Paste Layer to copy the group from the second project. If the mirrored bone names match, they should work for the mirrored actions that come in with the pasted group.
If you're pasting in mirrored Smart Bones Dials, chances are they will break, and you will need to reset their Actions. This is easy and not a big deal...just open the Smart Bone Action for the dial bones, delete the keys, and re-key them. Fortunately, the keyframes in the Smart Bone Actions inside the group items should be fine after pasting, so there is no need to worry about that. Just be sure to use Copy Layer and Paste Layer with the group, and not Copy and Paste.
I'm writing from memory, but I believe the above steps are accurate. Once you get the hang of it, it's not a difficult or time-consuming process. I'm making a video tutorial for this, which will go into more detail. I will let you know when it's up.
Alternatively, you can use Mult Rush's
MR Transform Rig Tool. This tool was made for this sort of thing but be forewarned that this is an advanced tool. I've used it to duplicate and mirror limbs in tests and had a few issues, but I'm pretty sure this was a user error on my part. If I figure out a workflow I can use, I'll make a video about it. Also, the creator, KuzKuz, made a video demonstration for the tool. It's got English subtitles and is informative. You can figure it out between the video, the info on the script page, and the built-in help. If necessary, I'm sure you can ask him for additional info, too.
I hope this helps.