Getting a good sear on scallops is simple.
First get a clear understanding of the science of cast iron skillets. Don't be confused by :"good heat retention". what it really means is "poor heat conductor"
They make cast iron cookware thick and heavy because thin cast iron cracks. Being thick, it keeps a lot of BTUs, being a poor heat conductor, it can't get heat up fast enough to sear both sides of the scallops.
So, when I have to sear scallops well, I use...