I think you're in a pickle trying to get the cables out the back of the stem cover then immediately turned 90 degrees down into the port in the frame.
As a possible solution I'm envisaging an inline pulley arrangement behind the stem, something like an Avid rollamajig, Problem Solver Travel Agent, etc, but looping up and over through 270 degrees not 90 degrees.
At the front end, maybe a sleeve of heatshrink would be neater than electrical tape? Exit the cable at about the extension clamp and it should have enough bend to tuck under the stem cover. Either that or slot a hole in your extensions, if you dare...
(I would imagine the aero penalty of doing this would probably be more than just the bare cables as they are now...)

I've never dealt with hydraulic bike brakes but I don't understand why noone uses hydraulic tube fittings to just make an instant 90 degree bend. Such as: https://www.mcmaster.com/51115k554 (I make no guarantees about this part or its suitability, it's just there for an example)


Or upgrade to Di2 (sort of pink)