The above description is very helpful it should give you most of what you need to build the deck. I’d probably work with curves and the NetworkSrf command to get the single surface I needed and then use the OffsetSrf command.
If as threedee has shown, you create what you need as a single surface, the OffsetSrf command with the solid option selected should give you what you need. Once you have a version you’re happy with just copy and paste it to the side and keep working on variations.
Experiment with the Rebuild command on both curves and surfaces. Play around with Untrim too so that you’re working with a square surface and utilise the curves extracted from future variations as you go along. Try to keep it really simple and keep versions of the deck as you go along so that you can move back to a different iteration to extract details and work on it without added fillets and holes etc.
Andy