But most of the movement is in the shoulder actor, not the collar:

Shoulder:

Twist -26

Front-Back -65

Collar:

Twist 15

Front-Back -10

Up Down -14

Looks plenty flexible to me.  ;-)

Here is the collar rotation set to -90:

Sure, I could probably improve that. And if everything else fails, create a JCM.

But that might create problems in other poses.

And the first pose looks actually more natural to me, anyway?

*

OK, I play along.

The collar twist joint on my David WM was still using a sphere.

I turned it into a weightmap, but I cannot make it look good beyond a limit of -50°/+50°.

 Otherwise, something will break and I'd need a JCM:

I'm pretty sure there IS a reason that DAZ limited the collar twist to -20°/+20° by default. ;-P

*

But seriously, I'm just afraid you make life unnecessarilly hard for you.

And then maybe give up because everything turns out to be just a chore.

(Been there, done that. Got the T-Shirt) 

;-)