I think you solved your own mystery there.
No one complained that O'Brien felt underused because he was only really seen during the rare visits to the transporter room and his other appearances were a bonus. And the rotating cast of helm officers made it hard to remember names or even care about who was in the chair this week. So there was nothing to complain about.
Owo and Detmer were there on the bridge in all the big dramatic moments with the camera making sure to show us their reaction, and they seemed to be Burnham's friends, so it felt really weird that they were basically just extras. It was a problem that was only really fixed for me when they were replaced with different extras that I didn't know.
I think Mitchell's been used better though; she feels more included, more of a character. I want the series to use her more, but I don't feel like she's being used too little.
They pay scale for regulars vs returning extras is different. Detmer had her PTSD journey in S3. Owo had "New Eden," but thatcwas about it.
Uhura, Sulu, Scotty, and Chapel rarely, if ever, had episodes centered on them. Chekov either.
The 26 episode Trek did more character development. They had more episodes and every week wasn't some galaxy level threat plot driven serialized mystery that everything is in service to.
SNW has done a better job than most new Trek in doing this. They had opening with different characters doing the log virtually every episode in S2. Episodic helps. Lower stakes helps.
But having said that, Trek is usually a big 3 and everyone else kinda fits in around that. TOS was obviously Kirk/Spock/McCoy. ENT was Archer/T'Pol/Trip. TNG was more of a big 2 with Picard/Data. DS9 was more ensemble, but I'd still go Sisko/Kira/Odo. VOY was Jameway/Seven after S3, though The Doctor has a good case for the 3rd. DISCO was Burnham/Saru/Georgiou.
A quibble here and there is fair. But that is how it just is. SNW is gonna center on Pike/Spock/Una. They, and the other legacy characters (Mbenga, Chapel, Uhura, Kirk, Scotty) are the focus. The only real new characters are La'an, Ortegas, Hemmer, and Pelia.
La'an has gotten the lion's share of development here. She has a bond with Una, and the Gorn, and a thing for Kirk via Tomorrow. Hemmer got his stuff with Uhura and the big sendoff.
In Lotus Eaters they did, seemingly give a nod to Ortegas never going on away missions. But this show is gonna center on Pike/Spock/Una. I get wanting a bit more of Ortegas, but at least she is there every ep. Scotty & Sulu were not.