Thanks Danny
I appreciate your suggestions. It will try to do it by boolean difference
If the shelling works for 0.5 mm for this surface, can’t it make “smart shelling” – instead of creating a thin wall – it would simply solidify the areas where thicker wall (like 1.5 mm) interfere with each other.

Another way I tried it:
I blew up the surface in 4 pieces, then shelled each and tried to join / boolen/union them.
It did not work.
Could you try that approach please?

Thanks
Olga