But my final goal is to be able to create a bill of materials on the assembly and to easily extract the material requirements through a macro.
Total surface area of thickness A and grade X sheet metal
Total surface area of plate of thickness B and grade Y
Total surface area of thickness C and Z grade sheet
However, I can't get all the necessary information on a single line.
The marker sheet 1 is on a line, but by default SolidWorks is in multibody and I therefore find my sheet metal information on the lower "child" line, not practical at all...
So do see a right-click on welded parts lists and then see if you have the little checkmark in front of Create a welded parts list and update automatically. See PJ
There is also the possibility via the mycad tools to retrieve properties of the sheet metal via smartproperties (I get the max length and max width of a sheet metal side by this means)
As you have access to these third-party tools, it's up to you to see the easiest solution for you.
I think the easiest solution would be to make a note as shown at the beginning of the post and link this note to a custom property that you can retrieve in your nomenclature.