I should probably add something: the back length likely won’t change, but the appearance of a short back and high tail go hand in hand. If you are picking birds that have cushions that sweep up to the tails right at the get go then the tails will be straight above the rear instead of flopping...
I wouldn’t recommend crossing your varieties. I agree that Abagail Wotjecki has nice birds so if you want to do the outcross, that’s fine, but these birds are actually quite nice and you could easily improve what you have. Just hatch a lot of offspring and next year set the goal to select higher...