The energy difference between highest and lowest value differs in each case.
-For l/d = 0.6, this energy difference is about 0.6.
-For l/d = 0.2, it is around 0.05.
-For l/d = 0.05, it is around 0.012.
The effect of the nucleus not being a point-charge becomes smaller as the size of the nucleus becomes smaller compared to the electron-cloud. Put otherwise: when the nucleus approaches the situation of a point-charge, the quadrupole term becomes smaller as well, leaving the effect of the monopole term, which you find using the approximation of the nucleus being a point-charge, to be dominant.