Re: Accuracy casting
A guy I fished with could only cast accurately if he stuck his tongue out-- like michael jordan shooting hoops. It was very funny and we all made fun of him, but he caught fish....
It's funny you mentioned sitting while casting, because that eliminates another suggestion. When casting short distances for accuracy i stand right foot forward (same side as casting arm), when going for distance, right foot back in a more open stance. Same principle as darts vs throwing a football.
It may be also that in the final forward cast you dip your rod a little far forward without realizing it, which tends to open up the loop. I have a tendency to do that, and find like FrankB suggested, aiming a little high on the release 2-3 feet over the target helps to keep the loop tight, at least for me.
I think if you're using a weighted bugger, it should have enough oomph to layout with the mono since the leader doesn't seem too long, but an 8' knotless bass leader with a heavy butt might help too, and will help layout the fly. They're built for throwing heavily weighted flies or large wind resistant stuff like poppers to get them to lay out straight instead of doing a "dying Quail" in the air at the end of your cast.
If you want to experiment a bit, try a 3 1/2 to 4' section of 20-25lb mono as a butt for your leader if you have some lying around, with a small perfection loop at the end. Tie a 2 1/2-3 foot section of 6lb as the "tippet" to the small loop with a loop to loop, or directly with an improved clinch. If that seems to help, you might be happy with that set up and call it good, but you may find that going to a furled or bass leader may work even better for you if the heavy mono butt seems to make a difference in getting the fly to layout straight.
I have a 5 SP rod too, it's a real sweet rod. And have been thinking about the TT line to match with it. Glad to hear you like it with the rod.