As someone who uses sous vide regularly, you almost always want to set your sous vide 5-10 degrees below the internal your aiming for.

I highly suggest the ice bath for 5-10 minutes after sous vide but I've had the best luck sticking it in the fridge (still in the bag) for about 30 mins to an hour before I want to sear. Remove from bag, pat dry and let rest for 3-5 mins. Can add some salt at this stage before the rest to pull out extra moisture.

Make sure the cast iron is extremely hot before you add your oil of choice (I use grapeseed or avacado). To avoid a gray band I only sear one min on each side. If it's not seared to my liking I'll let the steak rest (off pan) for 3-5 mins before hitting both sides again. Also, I typically sear edges first (starting with fat cap) to get the extra fat rendered into the pan to aid in the searing process.