Yep, a 10w is about right, depending on the rod and rod brand of course. As you say - a 12wt can be a pain to cast for any length of time, but you do need some rod strength to control the dying stages of the fight around rocks and weed. A 10wt is strong enough for this (just).

It's the big difference between a sand flat and coastal rocks. A kingfish can be eased into shore over a sand flat and some good sized kings are landed on light fly rods over sand flats as a result. Around the rocks you need a bit more control.

My opinion anyway