i think i might actually be understanding the source of the camera jitter problem, and it was a little bit right-under-my-nose.

first of all, after a bunch of consideration, i set the physics timestep to 1/60th of a second instead of Unity's default 1/50th of a second. the idea of a standard 60Hz monitor not even getting a physics update often enough bothers me without thinking about all this headache regarding the camera. of course, aligning the physics timestep with my normal framerate timestep does make the jitter disappear 99% of the time, but it is not a real solution to this camera issue.

so the camera issue is because i attached a rigidbody to the camera and have been using its velocity for transform changes. obviously, setting a rigidbody's velocity means its position only updates on a physics timestep. i managed to completely overlook that in my interest in having the camera be a real physics object. so i'll just remove the rigidbody and switch to only transform.position changes. easy enough?

knock on wood, this might not actually be the end of all of my problems. once i make sure the camera's follow position AND look at position are both being properly updated in Update and only Update, there will be still be potential inconsistencies between FixedUpdate and Update, so we'll just have to see.