After testing your render engine I have to say it promises a lot, congratulations.
When I was testing your materials I miss some nodes that are implanted in almost all renders engines with node materials. They help a lot to create materials efficiently.
I don’t know if you have already thought about implementing them, or I haven’t seen them.
But I just wanted to list them just in case.
-Gradient Shader node with differents options (linear, circular, box, etc..)
-Ramp node, to determine how the gradient of input ist mapped.
-Mix texture node, with multy input and alpha channel option for every input, also blend option (multiply, screen, overlay, add etc…)
It would also be good to implement the option of rounded edges in the materials.
To avoid sharp edges of the geometry and give more realistic small bevels, without having to modeling it.