And then transmitted to the lower planks through the internal drifts. Normal design for the time. I get to study wooden rudders in the boatyard below. It's both fascinating and terrifying.
The techniques varied widely but worked extremely well, and still do.

The terrifying part is how their failure can be catastrophic. I think that is exacerbated at times by owners of boatyards not experienced in their construction. When the internal drifts wear to a point, they can all fail at once. When these rudders were the norm, they knew what to look for, that was standard maintenance.

I think new rudder construction in composite materials is an improvement, mostly due to the internal skeleton, including your plywood/glass idea.

Even some of the new wooden boats being built are having composite rudders built. Composite rudders are not as likely to fail catastrophically, more likely to show signs of problems first.

I would have done something similar had mine not been in such good shape to repair.