I was thinking of a model from which you could compare differences or something.

And how could we possibly measure millimetric differences in actual orbits in order to compare them against your perfect model?

And for docking, I was still thinking of the Kerbal Space Program. Not an actual physical spaceship.

So you are going to calculate the positions of the simulated spacecraft within your game, to millimetric accuracy, despite that you would need a screen the size of the Earth's orbit around the Sun in order to notice the difference?

PS. Also, the sun happens to be almost perfectly spherical.

I can only assume that you state this in connection with the Sun's gravity?

The variability of the Sun's gravity is not due to the shape of the Sun; but rather to density changes caused by the convection currents that cause the constant churning of the gas.

Look up the Variability of solar IRradiance and Gravity Oscillations (VIRGO) instrument on the SOHO spacecraft; reference g-mode waves.