I didn't notice that the shanked bit was a Myler with the barrel middle rather than a broken middle such as is on a Tom Thumb. In my experience those are too unsteady in the mouth and will often create an anxious horse or a head tosser. But that bit looks much more stable and would be more like a Western grazing curb but allow you to use one rein at a time for signals.
The curved shanks will signal the horse and give them a chance to respond before the curb kicks in, so that's nice. It appears thin enough to be comfortable and there is no high port to hit the palate. It is probably milder than many curbs. The curb action would be more clear to the horse with a curb strap on it.
You can use direct reining on any bit, of course. However, my bias is that when direct reining, the signals become less clear the longer the shanks are on a curb.
My bitting system is like Freia's, starting with the mildest and progressing up. If a horse was too strong in a Kimberwicke I would go to a very short shanked Pelham next for a little more leverage (have not run into that yet).
BSMS, I hope the Kimberwicke will work well with Mia. On my most excitable Arab I use it on the lower slot, but I keep the chain loose so it does not engage unless I pull back on the reins to a certain degree. That way I can keep a light contact with the reins, which my horse likes. But when we run into a herd of elk on the mountain and my mare wants to gallop home to be safe, the chain can be engaged and she will listen to it.
I have not used a Kimberwicke with an unbroken mouthpiece because the way I ride with the loose chain would make the bit a mullen mouth most of the time. A mullen mouth is milder than a double-jointed snaffle even, so too mild for my mare to pay attention even when she is not excited. Of course if you ride with a looser rein and keep the chain so it engages whenever the reins are taut, then your horse will be working off the curb all the time and the unbroken mouthpiece may work fine for you.
It just depends on whether you keep up a conversation with the bit, or whether you use it more for stop, go, and turn. Or if you use neck reining in the mix.
If you haven't used a Kimberwicke before, pay attention to whether the metal clasps on the end of the chain are poking your horse where they attach to the bit. I cover mine with electrical tape. Also, once you know how long you want your chain to be, make sure to close the ends. I've been on horses where one side of the chain flew off while galloping. It hits the horse in the face as well as making it difficult to stop. No, not my horses, and yes, more than once.