Check some of the YouTube videos on how to set up an Egg for low and slow cooks.
Oops, looks like you already did that.
Check the Kamado Joe YouTube videos for how they set up the KJ for low and slow. I'd bet that the same techniques are applicable.
The recommendation for the KJ is to open the bottom vent about the width of your index finger and turn the daisy wheel to where the holes in it are about halfway open, at a given point when you are trying to reach your target temp.
If you only open the bottom vent the width of a quarter, I'd be afraid of either snuffing the fire completely out, or getting a lot of acrid white smoke from smoldering partially lit coals and or wood chunks.
I have a Kamado Joe. But no doubt, some of what applies to it, likely applies to your Egg as well.
The "problem", as I see it, with ceramic Kamados is that they are very efficient. Very insulated. This means that over time, you get a heat buildup inside of them because they are ceramic. And because they are so well "sealed". So IMO, they are too efficient for the casual or occasional user to consistently dial in for low and slow.
They lose very little heat through their skin, unlike metal cookers, and the good Kamados have less air leaks. You close the lid on your Egg, and you don't see even so much as a peep of smoke coming from around the lid if it's gaskets are good.
When that heat builds up inside, and at the same time the coals are still burning and in turn igniting other surrounding coals that were unlit, well then the rate at which the old coals burn out, vs the new coals igniting, and the already trapped heat dissipating, all become a delicate balancing act.
If heat isn't lost, as fast as it's gained, well then it's easy to see how it builds up inside the Kamado, and the temps rise. And you end up exactly with what you are witnessing.
Knowing what I know now, and seeing what I've seen in my own KJ, which is very similar to your Egg, I'm doubtful that you'll be able to get it to "hold" for any significant length of time to much lower than 250° at the dome.
I've never seen better than about 240° and that wasn't for longer than about an hour, maybe two.
I got my egg this summer and I am doing my second pork butt and I just can't seem to get the temp to stay under 250.
If you're seeing that in a ceramic Kamado, and can get it to lock in at the 250° range, be happy.