The fibreglassing work is the bit I'm probably least knowledgable with on my build so you'd be better off listening to other people on that subject!
But I know what you mean about throwing so much away, I've re ordered rollers and brushes twice already, sometimes they get used for 2 minutes then binned!
Youre right I have gone for just one layer of 450 csm (+ a layer of tissue), originally I thought 2 but when you calculate the cost of 2 layers on a 28ft boat...
I've used 1/2" ply and its well screwed down so I figured I was wasting money as its only purpose is as a waterproof barrier and base for the topcoat.
I used a huge amount of resin tho, 50kg inc all the other little bits. The main reason for this is because given my experience, I let the resin go off on top of the 450. This allowed me to sand the bumps out before putting on the tissue which turned out to be a good idea
It just meant I resined the whole deck twice more than I needed to had I beeen a proffeshnial.