[LH Lead-Foot]

No photos of rear caliper yet, but every post is good information. As a retired ASE Master tech, I grew in the age or rebuilding components, not opening boxes.

When it comes to rear disc w/E-brake, the piston screws in for new pads or out for service. Working the lever to advance the piston(s) toward the rotor is required. Yes, it take a long time to get all of the air out the line to both rear calipers, but you indicated 1000 - 1500 lbs. with little fluid. You will not get as much as front in normal conditions, but you have all aftermarket components with a knob on the proportioning valve. You're on your own there with customer service on speed dial.

I don't know what the answer is for your magical mystery tour of 2018's winner of the most expensive/longest brake job... and still no rear brakes.

Your pressure should work and the rears take little volume to move the piston(s).
Pressure bleeder, yes! Buddy pumping the pedal, press twice, then hold. Open blender screw, pedal drops. Repeat until no more bubbles. Easy right?

In short, everything should work! I know as well as anyone, that to go fast, you need to stop very well. But WoW, surrr do look perrrty!

Glad I used all GM parts to get disc brakes on my 1967 C10.