Getting averaged value of grayscale pixels on object

Hi, first of all I tried to look for an answer or anything that could help but didn’t find what I was looking for atm. It became very time consuming so I’m asking here, sorry if this is already explained somewhere

Here’s the thing :
I’m trying to get the averaged value of a gradient noise paning on an object using wordspace coordinate. It’s all about vertex offset, I’d need ALL the vertices of my objects be offseted in Z simultaneously. So it would lift the entire mesh up depending on the value it gets from the gradient. But right now it’s deforming my mesh and there is a wave effect on the objet while I want the wave to affect all my instances in the world while they remain looking solid

Do not hesitate to tell me if I’m being unclear

So I thought about a way of setting it up the way I want it, by averaging the value of the noise inside my instanced mesh. But I have no clue how to do so or if it’s possible without being overly expensive in instructions.

I came across this post :

Which could be a lead but right now I’m using procedural noise and not a texture. And I’d like keep it that way for now. If i’m using a cube I guess I could just sample 4 pixel in the corners and get the average value, but since the noise is in world space does that sill work using the cube UVs ? Sorry if it’s a noob question, I can’t test it right now. Also I don’t want to use a cube so I’m not sure how to proceed with a complexe shape

That’s it I guess,
It would be awesome if someone arround here has any idea that could help

Thank you!