You would not need a wedged half dovetail if the king sits on the 18' tie with a post under it, at the first level. This configuration is not a true truss. The king is in compression, in this case not much thrust from the rafters on the post either. I would want through tenons and a heavy peg or two at the post/tie connection regardless of the lack of thrust from the rafters.

The width of the king may have importance due to the joinery at the peak where the rafters connect, you could flare it to give added wood at the peak and the lower half necked down to 7x7. Lay it out from a snapped line.

You wouldn't even need braces from the king to rafters,(the rafters are acting as bracing members if I am not mistaken) but they are fun, so why not. Just us a 45 degree angle from the king. I see the pitch is 10.5, perhaps square the brace from the bottom of the rafter the angle would not be a 45. Windows usually get in the way, so they may have to be on the shorter side.