I have no exact answer but the earth’s mantle makes up approximately 84% of its volume and has temperatures ranging from 500k to 4200k. So I would estimate the average temperature to be in the upper bounds of that range.
So that figure is just the mantle, no core or surface? I agree it would be near the top, if not above. The heat from the core would easily offset the loss of heat from including the surface.
Yeah, the volume of earth is <1% the crust, ~84% the mantle and ~15% the core. So the surface temperature has basically no impact on average temperature.