Captain Kathryn ENT is my favorite version of Trek. You are right about the secondary characters fell into the shadows as the show progressed. But the last two seasons are some of the best Trek ever made. The Xindi arc only had a few stinkers (Hatchery) and season 4 was just great, Augments arc, Vulcan arc, mirror episodes. Except for TATV, which really sucked, it was a great season.