This sounds like a mob tagging issue.

While shield lob is good for tagging individual mobs, it's generally better to pull clusters of mobs with flash/unleash as you run through them. If you need to tag two different mobs within the span of one GCD, use a regular GCD on one and an oGCD (like Spirits Within) on the other. If you need to pick up multiple mobs that are scattered about, predict how they will move and intercept them en-route to your healer.

Don't worry about marking targets to attack. The priority target generally ends up being the one with the lowest health after everyone settles into place. You can indicate this non-verbally by leaning more of your dps into a particular mob early on in the pull to drive their HP down more quickly than the rest. Outside of this, you want to establish threat on everything equally until you get a sense of who's on what target. Marking is more important for critical moments when you need to catch a player's attention (i.e. bees/final sting).

The only way to set the pace of the run is to be ahead of the group. This actually maximizes the amount of time that you have to set-up each pull. You should have a good mental map of where each cluster of mobs is going to spawn, how you're going to tag each set, and where the group's final position should be. The more clearly you have this planned out in your head, the more fluid it becomes. You can make an individual pull as small or as large as you like depending your level of comfort and safety, but do it in one seamless motion until you want to indicate to the group that it's time to stop and dps. When you run the same instance 20 times, the movement and positioning on each run should look nearly identical once you've decided on how to optimize it.