I'm annoyed with the 40kg weight, as it stops me from planking a full inventory of (oak) planks. I have boots of lightness and spotty cape and have -6kg when I start with empty inventory. Is it possible to get this to -13kg?

I dont think so but Penence gloves can help this (correct me if im wrong) but I think they reduce 5 weight

get your hunter higher so you canwear the spottier cape. This one gives 2kg extra (5 vs 3) weight reduction. Gloves of penance (from barb assault) gives gives 4kg reduction. This would give you a reduction of 13.5 kg.