My own set of Panasonic 14-140 f3.5-5.6 questions before I take the plunge...

geepondy wrote:

- Reviews I read mentioned shutter shock but these are older reviews with G7 and G7X cameras. Do you think I would experience this on my GX9?

No.

-Just how prevalent and serious is this issue of not allowing a shot to be fired in low light situations? That sounds like it could be quite aggravating.

I haven't heard of this as a lens problem.  If your camera setting for shutter release is set to "focus" priority, the camera will not take a photo until AF thinks it has found focus.  This can be an issue in low light with a lot of lenses.

Would the 14-140 be giving up anything in sharpness (as well as color and contrast perhaps even more important), compared to the 12-60 and 45-150 within the overlapping focal ranges?

As to the 45-150, no.   I haven't used the 12-60.

The Leica 15mm lens has me drooling and makes me wonder just how much sharper the Leica 12-60 f2.8-f4 lens would be over the Panasonic 12-60 but in reality whenever I'm out doing nature walks, I'm constantly swapping between the 12-60 and the 45-150 so in practicality the 14-140 makes more sense. This would be the last lens I would buy for quite some time although if I find necessity dictates, I may get the 12-32 pancake as it's cheap.

Another option you could consider if you want top image quality and want to swap lenses less often is the Olympus 12-100.  It's bigger, heavier, and pricier than the 14-140 II, but its image quality is amazing, it's constant f/4, and it's weather resistant.  I bought a used one for $900 to replace my 14-140 and I'm happy with that decision.  It doesn't go to 140mm, but it goes to 12, so there's a trade-off there.