Help regarding specific point during Civil War/Season Unending

Hello, does anybody know the cutoff point where you can no longer trigger the season unending quest while doing civil war? I’ve just finished clearing the fort after saving the imprisoned imperials & am concerned I’ve missed my chance to play out the season unending quest. I’ve yet to report to general tullius & gain the rank of legate yet, does anybody know if I’m still able to trigger the truce quest line? Or have I already progressed it regardless of whether I report back to the general? Thanks in advance