You did REAL good work2much! You took the time to evaluate all the options and didn't cut corners. I went the diode method for my lighting, found a kit on e-trailer that I could understand the instructions to. Your method sounds like you found a solution you could understand as well and that is always good.
I have a Super Hitch and therefore a lower receiver hole to route my extension. If you had one, you might not have needed the drop.
Excellent results!
Here's some suggestions when you test the tow;
Find a wide open area and have a buddy ride in the bed of your truck. Have him observe the whole assembly as you maneuver turns and braking. I had to make slight adjustments to my turnbuckles and tension on my brake cable. In fact, I added a small turnbuckle on my brake cable for fine tuning. It's not feasible to adjust the length of this cable because it will stretch as it gets used. Test it going over curbs and troughs where the angle between the truck and the towed will be at it's most. Try braking during this and see how your towed brake pedal reacts. Try turns and braking as well.
I want to mention about cable routing. Your READY BRAKE towbar has the loop already but others who read this may not be aware of what they are looking at and what it does.
I welded a loop at the ball mount end of my tow bar. This is to route the brakeway cable, the brake cable and the light wiring cable. By routing all these cables as close to the pivot point as you possibly can, the cable lengths won't be too long or too short as you make left/right hard turns. I found that before I did that, on turns those cables would sag, then when I pulled straight again, they would hang up on various parts of the tow bar. READY BRAKE now sells this as an option with their main product. It should be standard as a safety feature! They stole my idea!!
Here's a photo from their website showing the cable pivot right above the tow bar pivot point.

Here's a link to READY BRAKE's turnbuckle for cable adjustment as well. (I couldn't capture it as a photo and post) It also shows the cable pivot loop idea as well;
TURNBUCKLE brake cable adjustment