I came upon this progression: C#Maj7 - C7 - Fm7 - D#m7. The song resolves later to G# major, which I believe would make this a IV - III - vi - v progression in G#. My question is regarding the D#m7 here. Usually it would be a major V, in the form of D#7, but here it's a minor v instead. I am very unfamiliar with minor v chords, and I'm surprised to find it works so well here, better than the V7 imo. From what search results I could find, the function of this D#m7 seems to be a secondary supertonic, driving movement towards the secondary dominant of the IV (G#7, aka the I7) making this a ii/IV. There doesn't seem to be a I7 chord here though. I would guess that the secondary supertonic has enough pull towards the major chord two semitones below, by way of leading to its secondary dominant in a ii-V fashion? Am I on the right track with this analysis? Couldn't find much info/discussion about secondary supertonics!