Assuming a standard setup up all you had to do was place an IK bone into the correct camera driven position, and then use any of the IK solvers with a variable alpha.
Try something similar to this, but move the IK bone, then have the solver address it.Unreal Engine Character Hand IK placement - YouTube
Also, for a proper first person view, perhaps you’d have been better off with an aim offset, especially if the flashlight is one handed.
For that, all you do is pop out single frame animations of the various angles from a DCC just like the aim offset. same idea - and it would work quite the same, possibly much better then IK.