You don't understand this point, do you?

The way insulation works is it slows down the conduction of heat. That way, the effective temperature of the radiating surface is lowered. The Stafan-Boltzman law then steps in and shows how much the radiated heat is reduced by.

If you don't understand that, try this...

As a first approximation, you might assume that the skin of the spacecraft is cooled to ~ 10 K (the temperature of uch of interstellar space) or you could use 0 K, the difference is trivial.

You desire an internal temperature of ~300 K. So the insulating layer must sustain a temperature difference of ~300 K.

Heat Flux Q per unit area = Conductivity * delta T / delta H

If we assume an arbitrary thickness of 1 metre, then Q=300.C

Total heat loss E = Q.4.pi.(r^2) or Q.pi.(d^2)

so E=300.C.pi.(500^2)

The question is, what is an appropriate value for C?

Solid rock has C~2 W/(m.K)

Spun insulators have C~0.01 W/(m.K)
Multi-layered insulators with reflective layers and vacuum layers have C~0.001 W/(m.K)

So, E~0.3.pi.(500^2)

e~ 250,000 W

250 KW - the power generated by a large automobile engine.

You can increase the thickness of the layer if you desire better insulation and less heat loss

Alternatively, you go to a larger sphere with less surface area to mass ratio and things get simpler.