Damn, man. I've been sleeping on the spring bone add-on.
By far the easiest jiggling effect I've found, and I ignored it up till now because I assumed it needed a better rig to be worth using. But in this case, it works quite nicely.
It does have some limitations and quirks. The way it works is: You set a bone to be a "spring" bone, and if you move any of its parent bones, then it jiggles as it follows that motion, much like a loose spring.
In other words, you can't get jiggling without motion, which makes it difficult to use this add-on to simulate "impact" jiggling, like smacking an ass cheek or something. The main reason I have Julie's entire body moving up and down a bit here is to provide the motion needed for the jiggling.
But boob jiggles aren't the only use-case for spring bones. I set the thigh bones in her legs to be gentle springs as well, giving the legs that subtle swaying motion. I also created a new custom bone that I weight-painted to her ass and thighs to give the skin some additional jiggling, though it is also pretty subtle.
Here's a screenshot. All the spring bones are colored green, so they aren't being animated manually.

Unfortunately, I have to bake the animation in order for the outline rendering to match the spring bone motions, and it seems that this process can have inconsistent results if I'm not careful. But overall, I love how easy this tool is to use, and I'm thinking about trying it on some of my old animations to see if they can be improved.
In any case, enjoy! ~💕🎶