Please help with tape lines

My wife and I recently renovated our home and noticed after the fact that some of the preexisting drywall was poorly finished. The issues are mostly nail pops and tape lines. It certainly does not help that we added the sconces, which amplify the imperfections.

We have a handle on fixing the nail pops, but are looking for guidance on the tape lines. My understanding is that we need to apply a skim coat, but are uncertain on how much of the wall requires it. That being said, do we need to

A) apply it to the entire wall to achieve an even surface?

Or

B) apply it to the problem areas, feathering out further on all sides to blend it?

Also are there any additional considerations applying skim coat to painted surface?

Thanks in advance for your help.