Koujo Denka no Kateikyoushi‘s second arc differs from the first in that the focus is far more on the girls than the protagonist and how they grow (or, in one particular case, regress) in his absence.
It’s also notable for ending each of the first three books on a cliffhanger, a pretty painful state of affairs I’m extremely glad I sidestepped by waiting for the whole thing to be released before reading.
Overall I’d say these four books end up a near-perfect continuation/expansion of the series with pretty much nothing to complain about besides the above cliffhangers. That said… I do not see any way this harem situation could possibly be resolved amicably. Even if we were to dismiss the younger girls as simple crushes, that still leaves four (two duke’s daughters, the merchant girl, and his foster sister) who aren’t particularly close with one another in the running.