yes, the thing with the cardboard is recommended more often (can't be too wrong), but it doesn't really make sense to me because the water between wood and cardboard (presumably) doesn't disappear faster than between wood and galvanized steel. ...
Tar paper and wood bond together. When there is enough press pressure. You know cardboard gaskets in engines? For example, at the water pump? It seals even without tar, that is waxed cardboard.
I think thick VA washers would look best. Maybe I can glue them over the pre-drilled holes before laying down the wood...?
A lathing goes on the steel construction, then the wood on top (the lathing construction can be placed on rubber pads. Of course, the laths must be screwed together so that they do not shift). Extra spacers go under the decking boards so that the boards do not lie wet. You can google that. All other variants are amateurish. Cardboard is total rubbish. You can watch how it rots.