It is very strong!
I hope the 30 mm was a typo and you meant 3 mm. Even when steamed it would be awfully difficult to bend 30 mm thick plywood. But, plywood that thick is laid up in molds for many projects. The individual layers of wood veneer are very flexible, and can conform to curves without using steam or great pressure. The veneers are coated with glue, set in a form, bent to shape, and left in the form until the glue sets. There used to be a lot of furniture make this way. Now I see that it is the technique used to make skateboards,

Using 2 or 3 layers of 3 mm plywood, and laminating them with glue would give you a better result than using a single piece of 6 mm plywood. The stiffness of the ribs depends on the orientation of the wood grain. With 3-ply plywood, the middle layer is going at a right angle to the 2 face plys. That makes it more flexible, and not as stiff a rib material as solid wood of the same thickness. The flexibility might be an advantage, though, giving a less fragile hull.

You can use thin strips of solid wood and laminated them to make frames and cockpit coamings. Any species will bend easily if it is thin enough. 3 mm stock is good to start with, but if you have sharp bends, use thinner stock and more plys to build up the ribs to the dimensions called for in your plans.

Hope this helps

PGJ