2 plys (layers) of 6oz will be enough, especially over plywood.
I did 2 plys directly over foam, and it is good for puncture and dent resistance, and adds plenty of strength.
I agree with Tony, use epoxy. It is more expensive, and you have to be diligent about your mix ratios, but it is a much better product IMO. Stays somewhat flexible and is less prone to cracking.
A small digital gram scale and a cheat sheet for batch sizes solves 99.9% of the mix errors. I use clear plastic cups to mix in (repurposed Dunkin Donuts iced coffee cups) and prefer them to solo or paper mixing cups. You can see the contents directly and can get a better idea that you are mixing completely. I keep a wrist watch with second hand in my work area and always time my mixing to the recommended 2 min mark (for West System). I use a paper plate zeroed out on the scale to catch any drips or spills (when using the hand pumps away from the scale there really aren't any), and on that paper plate I have my most commonly used ratios written boldly in sharpie. I converted the cheat list to resin weight then total weight of resin and hardener, that way I can pump my desired resin amount, check it on the scale (I usually have to add just a touch, like .5 gram... which is the resolution of my scale... to get it exact compared to equivalent pump strokes), then add the same number of hardener strokes and check for final weight on the scale without having to do any math. I usually have to add a tiny bit more of the hardener, too, to make it exact. I'm sure the pumps would be close enough, so long as you count right, but having the scale is a good check that you have gotten the correct number of pumps, and lets me get the ratio exact. In my whole build I only had two batches not cure properly. One was too small of a batch for the resolution of my scale, so the consequences were also small. The other was a small patch area that didn't layout right. I think I may have reused a mixing cup and the batch started to gel too soon; I was able to pull that layup off before it got too firm, and redo it the next day.
Prior to my build I had very limited experience doing composites. Looked at videos of home built aircraft and cedar canoes, and talked to a friend with experience. I started with my tongue box (a smaller project) to gain confidence.
With 6oz I found the smallest radius I could wrap w/o concern of lifting (and thus creating air pockets) was about 5/16 inch. Perhaps a little less, 1/4 inch, if laid on the diagonal. I had no problems at all wrapping the 3/4 inch radius between walls and roof, so my suggestion is to use the largest radius that suits your design appearance.
If you want a tighter corner/edge, like at your door jambs, you can use what is called a flocked (or flox) corner. It is a bit more involved, but it lets you keep a sharp corner w/o leaving any sub-material exposed.
Here's a spot in my build where I started using some of those techniques.The method I used on my hatch edges (laminating the underside seal lip, then back gouging for the flox before laminating the side edge... which you can see the result of at the beginning of this post) was tedious. I used an improved method on my door openings and door edges which you can see
about half way down this post.