“Whats the reasoning for the Orthodox for continuing to use the Julian Calendar?”
Because that’s the calendar the Council of Nicea used when it fixed the calculations for the date of Pascha.
So does the Reformed Julian essentially correct the following?
The Gregorian Calendar was devised both because the lunar calendar had grown conspicuously wrong, and the mean Julian Calendar year is slightly too long, so that the vernal equinox slowly drifts backwards through Julian calendar years. This caused problems in computing the date of Easter.