qStart and qEnd are your beginning and end angles.
The rest is how it calculates where it is between them.
You should be able to change the swing by making those two variables further apart.
I would try to just add a tiny bit to one and subtract a tiny bit from the other over time.
To be precise you could check for every time it reaches one side and add 10 degrees to the other. I think this might end up looking very unnatural though.
void Update(){
if(qStart - (MathF.Sin(Time.time*speed)+1.0f)/2.0f) < 0.01f) //very close to start
qEnd += 10.0f;
else if(qEnd - (MathF.Sin(Time.time*speed)+1.0f)/2.0f) < 0.01f) //very close to end
qStart -= 10.0f;
}