I think out of the entire second tier cast, Doohan was probably in the strongest position, as his character seemed to strike a chord with people, and the production team found themselves wanting to make more use of him. Nichols was indisputably in the weakest position. Not a slight on either the character or the actress, but she was only being employed as a day player and whether she appeared or not was entirely at the discretion of the producers.
Except that, as I said above, Nichols appeared in one more
episode than Doohan did. Although it's true that she was rarely given as big a role to play.
As the movies progressed, the status quo of Shatner-Nimoy-Kelley reasserted itself, though the supporting characters had kind of finally become indispensible by that point.
Except that Sulu kept getting short shrift. His promotion-to-captain subplot was dropped from TWOK, and most of his big scenes (meeting his ancestor, stealing the helicopter) were cut from TVH. And even though he got his own command in TUC, that left him absent for most of the film. (Then again, Uhura was the most sidelined one in TSFS.)