Heavy leather should have zero effect on how constrictive the jacket feels and any jacket that feels uncomfortable and restrictive in 3oz leather would feel exactly the same if it was made in 0.5oz hide.
Breaking in heavy leather is a myth. If you can do this in a full plate metal armor and not in a leather jacket (and I sure as paint couldn't in mine) then there's definitely something wrong with the pattern.
Interesting argument, but I do not find that the analogy works. Plate armor is articulated with joints. It is fitted to the wearer, a true bespoke process. When you flex your joints in a suit of armor, it bends at the joints built into the armor, and there is no resistance, because you are not bending any material. You're swiveling two pieces of metal along a joint, typically a hinge or a swivel, or sometimes two overlapping but not directly connected to each other pieces of metal.
With heavy leather, you are flexing the material, and it stretches. Over time it does break in, and will mold itself to the wearer's body, becoming more pliable in the process. You can notice a difference within just a short amount of time, and the more that you wear it, the better it gets.
One thing I hear about CXL is how stiff it is out of the box. The best thing for it is what Aero recommends: do nothing, and wear it. A lot.
If you absolutely cannot handle that, then one thing I do not hear much, but I know works from firsthand experience, is to apply a thin treatment of Pecards. I did this once on a FQHH CXL Aero that I picked up secondhand, which I thought felt a bit dry. It was just as stiff as all the other CXL FQHH jackets that I'd handled from Aero to that point, but after taking a thin coat of Pecards, it became much softer and more supple immediately, and felt like a different jacket entirely. It's like the leather had been standing at attention for a drill sergeant's inspection, and had finally relaxed, at ease.
For people who don't like the stiffness, but already have the CXL, this could be a much better solution than getting rid of the jacket. But if you're buying new, then consider Vicenza or Goat, or one of the other leathers Aero works with, like their Jerky or Badalassi, etc.