Just trying to present options. I also try to do things better every time I do them.
With that being said…
I’m not a fan of wood screws. I would through-bolt it.
What I think is important in building a structure like pictured:
The horizontal beams will experience a purely bending load, supported by whatever fasteners used to connect the horizontal beams to the vertical beams. The load is transmitted from the horizontal beams to the vertical beams through the fasteners. In the case of the horizontal beams, this load is perpendicular to the wood grain.
The vertical beams beams will be in tension, but will experience a bending force as the horizontal beams start to deflect. The deflection of the vertical beams doesn’t concern me as much as the loads transferred from the fasteners into vertical beams. This load is parallel to the wood grain. My concern is that the wood will split.
The composites will take the form of any shape in which they are supported in during the curing process. Any distortion of the press will allow distortion of the mold. All the time spent building accuracy into your molds will be lost if everything flexes and distorts in use. Ultimately, maintaining the shape of the mold is most important to me.
Accuracy of the bottom of a snowboards is the most critical. I wanted the bottom mold to not distort at all. Ultimately, the shape of the top of a snowboard is irrelevant and was dictated by the shape of the core, which was dictated by the flex pattern.
Wood is cheap, I see three vertical beams on each side and two horizontal beams. I would add three more shorter horizontal beams to fill the voids between the existing two horizontal beams on both the top and bottom. This is not for load bearing purposes, it is solely to prevent flexing of the press, which is translated into the mold.
For assembly, I would glue and bolt everything together. Use a lot of Titebond III wood glue on every joint that doesn’t need to move. Make sure that the entire surface is covered with glue. After the joint is made, the excess glue is easily cleaned up with a wet towel. It is better to have excess glue squeeze out than it is to have voids. Lightly clamp everything together so you don’t squeeze out too much of the wood glue.
I believe it would work best to drill and install the fasteners after the glue has cured. I would use 1/2" (12mm) minimum, 5/8" (16mm) preferred, bolts with a shoulder long enough to pass almost all the way through all five beams. I wouldn’t be the least bit upset if you opted for 3/4" (20mm) bolts. I’m suggesting these large sizes, not for load bearing purposes, but for support against the inside of the hole drilled through the beams. Surface area is the key here. The smaller the fasteners, the easier the wood will split.
Try to drill the holes to achieve as tight a fit with the bolts as possible. Lastly, I would apply ample amount of wood glue or even resin inside the bolt holes upon assembly of the fasteners. This is to provide maximum surface contact between the wood and fasteners. Apply multiple coats to account for absorption.
I would try to avoid using fully-threaded bolts or all-thread, threads do not make a good load bearing surface. If that is all that is available, you can install steel tubing as the load bearing surface and pass the fasteners through the tube to provide clamping force only.
Good luck with your build, let me know if I can help.