"the slide doesn't stay open on the last round fired" - Have you tried more than one mag ?

"Is this pistol tuned for a specific weight bullet and powder burn rate?" - Have you tried factory ammo ? Maybe too fancy too soon. I think you'll be getting replies that it "eats anything".

Wolff says:
RECOIL SPRING
For Use in:
39, 439, 539 ,639, 3904, 3906, 3944, 3946, 9mm
59, 459, 559, 659, 5903, 5904, 5906, 9mm
5923, 5924, 5926, 5943, 5944, 5946, 9mm
659, 909, 910, 915, 9mm
Reduced Power...: 8, 10 & 12 Lb.
Factory Standard.: 14 Lb.
Extra Power........: 16, 17, 18, 20 & 22 Lb.
I also think you'll be getting replies that racking a 5906 should not be a problem at all. With pistol empty of course and mag out, cock the hammer (decocker in up / fire position) and then try racking - it should be easier since you are not fighting the hammer. If it is still "across the chest effort" and since you are experienced, I would begin to suspect your recoil spring may not be factory standard. Even then, I've not found much racking diff with 16# or 18# recoils.