I will state, I am not an expert on internals of a FRP spar.....
My knowledge.....mostly wood laminations (not including balsa at the root for spar) with glass or CF for ultimate strength.
Further out, yes, maybe balsa for reasonable strength and lighter weight. Engineering basic is...."the more you make 3 dimensional, the better...", thus Origamy works with thin paper.
There is always diminishing returns...eventually you get past one engineering segment and start to lose on others....


I will let real engineers (ME or aeronautical) to dispute my comments.