Good questions. And welcome. I don't know that there's any efficiency with an SJ or a JJ, more likely a matter of convenience and portability -- but I don't even own one, so I'm just presuming.
The WSM is a fine smoker and has some distinct advantages, such as capacity and the ability to do low-and-slow cooks for a long period of time.
Basically, you already have all the grills that you absolutely need. But if you stick around, you'll come to crave kettles of certain sizes, types, colors, year of production, etc. And you'll maybe want multiple kettles because you'll want one dedicated to pizza, one dedicated to slow cooks, one dedicated to rotisserie cooks, etc. And you'll start wanting the various accessories, like the pizza cooker, rotisserie, rotisserie attachments, slow-n-sear, vortex, remote thermometer and much more.
Good luck and post some pics of your cooks and kettles!