Something like

my.pan += clamp(ang(offset_angle.pan - my.pan) * 0.5, -25*time_step,25*time_step);

maybe? If the last bit of turning should be smoother again, take the "time_step" out of the clamp instruction again and put it at the end of the line.

Alternatively or on top of that, you can separate move direction and the actual pan value. I do that every now and then, too, when I for example want to move an enemy plane on a rather tight path but still want it to look smooth. Something like this:

my.skill10 += clamp(ang(offset_angle.pan - my.skill10),-25,25)*time_step;
my.pan += ang(my.skill10-my.pan)*0.2*time_step; // or some other clamp thingy too
vec_set(temp,vector(15*time_step,0,0)); // the move speed
vec_rotate(temp,vector(my.skill10,0,0));
c_move(me,nullvector,temp,...);