It's one of the biggest ones.

My understanding is that there are still some others that need to be figured out, like "The physical structure holding the thing getting the everloving shit bombarded out of it by high energy neutrons and causing it to degrade and fail over time"

Computational electrodynamics.

We need to design better geometries for reactors so that we can channel the plasma flow to better improve containment while quickly removing wastes from the highly compressed energetic region where fusion mostly occurs. If we could do that, then that mostly solves the issues from reactor degradation and significantly improves efficiency.

The trouble is that at such high energy levels the plasma flow is relatively chaotic so high fidelity models are required to predict the behavior accurately.

Is this really a requirement, or a nice to have kind of thing? Commonwealth Fusion Systems are using a pretty basic design and seem pretty confident it will work