Sync values from one field to others in fieldArray

[y-wertz-priomold]

Hi,

Problem

I'm using field array to track multiple positions in an enquiry (each line is a field).

On the left I have checkbox to enable sync from one field to alle others that are enabled.

What I want

If the user changes "Material" in Pos 10 I want to automatically set Material of Pos 20 to the same value (if checkbox is enabled).

Thoughts

I wrapped my head around using watch, but using watch I only know some value of the form has changed and not which value has changed.
Using watch with a specific field name is not possible because of the parent field array.
For example the name of Material field is pos.${index}.material.

I tried making a onChange sync callback at the parent / form level which could be passed to each field, but even accessing the field dynamically seems to possible.

 const syncPositions = (posIndex: number, fieldName: string) => {
    const changedValue = getValues(`pos.${posIndex}.${fieldname}`);
  };

I'm getting this typescript error (which make sense because passing a string can be an invalid field name)

Help needed

Anyone ideas how to make this work?
Thanks!