think if you are going to invest 100 hours in building a boat, the cost of okoume is not much. The other thing to consider is that it is about 10% lighter then luan and 30% lighter then douglas fir or baltic birch.
I have built small boats out of okoume, marine rated douglas fir and other stuff out of luan. Luan is way inferior in quality. The faces are very thin, the core is thick and has voids, it does not like compound bends at all.

Do not cheap out on the plywood.

I was looking at the design and I think if you are covering it with 6 oz fiberglass on the outside you can use 4 mm okoume for the hulls no problem 3 mm might be pushing it.

A 5 piece stitch and glue hull 12 ft long with scarfs, and no daggerboard trunks and with full sized templates I would guess you are looking at around 40 hours per completed hull. Add another 20 per hull if you are super anal about fairing it and painting it with awlgrip or similar.
Build the rudders out of 2 pieces of 1/2" baltic birch. Make a design and have someone cnc the shape, then glue 2 halves together, run full lenght Uni carbon down the middle of it on each side then glass on one layer of 6 oz cloth and fair it.

If you get someone to make the cores for the rudders, it should cost ~ 150$ for the cnc cutting. Everything else can be built.

Just my thoughts.

P.S. do a web search in multihulls world about 10 years ago a german guy wrote an about making a 14 ft cat that weighed ~100 lbs to sail in indonesia on vacation. He shipped the plywood precut and built it in 2 days in indonesia and sailed it around the islands for a few weeks. He used a windsurfer mast.

Cheers,
Eric