Just to amplify on your point about lithium: the tritium production function is critical. Every fusion neutron needs to produce more than one tritium atom on average, so that the reactor is sustainable (there are inevitable losses & tritium also decays radioactively) or even making excess tritium (to bootstrap other reactors). This is challenging b/c even in the best case each neutron can produce maybe 2 tritium atoms, so there's not much margin. The lithium needs to comprise most of the material surrounding the plasma, limiting the fraction that's available for other functions (structural supports, heat shielding, cooling, plasma control & heating systems, sensors, etc).