Ah, you're right, but you're not talking too much about the emitter when you show plank, just about the EM field itself (and if you do it somewhat rigorously you at least mumble the words ergodicity/equidistribution, which fail in dilute gases with quantum electrons for example, so you get emission lines) I'm saying classical electrons are not enough to explain emission fully even in solids (that being said I don't know much/anything about the thorium case specifically)

Yeah, you can derive some forms of thermal emission from pretending electrons are classical point charges. That doesn't mean doing it will always explain the real world correctly! That's why we need quantum physics.

It's true! Googling spectral emissivity may give you extra info.

Thermal radiation (both in gases and solids!) can be extremely simplified to

1. Electron gets excited, likely through atom/atom collision, either through literal collision in a gas or vibrations in a solid.
2. Electron comes back down and emits a photon

If your material has a nice continuous energy band (something that doesn't happen in gases, but happens in metal), the electron can get excited and unexcited without many restrictions, so you get a plank type distribution. If the electron is restricted to belong to certain bands (like in gases and some solids) emission will have to be restricted to those bands.

In bulk materials with large optical density (such as solids) you have to account for reabsorption, which happens preferentially at the emission frequencies, and the fact that your system is out of equilibrium starts to matter a lot. You get a mess instead of clean bands, but you don't get black body either!