after looking at this more, I think I can pinpoint where the camera starts to go wrong.
if you are moving left and you scroll the mouse to the right edge and off again, it does the abrupt movement. I think it's because the mainCamera is trying to catch up to the player, then when the mouse goes to the edge, it jumps to the lookCam.
it doesn't happen the other way (if you move left and look left) because the cam is behind the player or something along those lines... I can't quite figure it out! but I sort of understand it.
I think I need to take into account the player's speed for the lerp value maybe?
EDIT: oh its that clamp that is doing the really abrupt jump. but I think this is what I meant by needing an easing transition. its the maxdist that needs to be the max distance the player can be from the center of the screen (with regular cam). but when I set it to that value (around 200), then the camera is WAY too fast.
EDIT2: so then I lowered the lookCam speed to 0.5 which helps a little. but still its wonky.