If the writers weren't devoting time to develope main of the main cast, what good would having a secondary cast do? That's not fixing the problem, it's ignoring and using a temporary distraction in hopes of fooling the audience. That was Voyager's main problem to begin with.
Everybody seems to be avoiding the truth.
Yes, Voyager's writers weren't always the greatest.
However, many of Voyager's characters were underdeveloped because Paramount spent money to promote the show using Seven. Mulgrew & Picardo, the two divas were both fighting for equal screen time as her. Too keep the two of them happy, they gave them what they asked for which left no room for anyone else in the cast. The rest of the cast go shafted due to jealousy and ego.
I'll blame the writers.
I'm not saying it wasn't an issue that they basically turned the rest of the cast into the equivalent of CSI staff. It just happened to work out well for me because I favored those characters. But I don't think it was due to ego. I think it was partially due to the limitations of the characters set up very early in the show.
From the initial season, Janeway, Doc, and Kes had the most screen time. Episodes were devoted to other characters, however, given the nature that Janeway was the captain, she was ALWAYS going to have a high amount of screen time. Can't change that. It's silly to even try. Every Trek series has always devoted a good amount of screen time to the Captian. Doc was also always going to be a key because in enough of the episodes, he comes into play. Partly for his hologram nature and jokes centered around him. Partly because he was the doc. He was the equivalent of Data only in the role of the doc. Consider making Crusher a hologram or android in TNG and right off the bat, there's a lot of screen time. Crusher or a doc was in most every episode. Same for Data. Combine them and by default there's a large amount of screen time going to him because there's really no way around it. When it's not a hologram thing, it's a medical thing. Kes and Seven were his Mentor then Student in that order. He was crucial in fleshing out both characters and they were crucial in fleshing him out. Consider the relationship between Geordi and Data. This is the same here. A fair amount of screen time was devoted to it due to the fact that the doc was a hologram. Had the writers had the foresight to see that making the rest of the crew dislike the Doc from the start was narrowing their options, then others would have gotten more time with the doc and more involvement in his development over time for at least the first three years.
Mulgrew & Picardo openly admit and never denied going to the producers to have more screen time and larger roles in the series. It's comon knowledge that Mulgrew felt threatened by the introduction of Ryan and how they used Seven instead of Janeway to promote Voyager. Mulgrew felt as if she were being replaced as the main draw to the series. Based on those facts, neither Mulgrew nor Picardo felt as you do in regards to the amount of screen time their characters would receive.
Kes has never had equal screen time to that of Janeway or the Doc. Even during the first 3 seasons when the cast was a more balanced ensemble, Kes had lesser screen time compared to that of her co-stars. Due to that, IMO that's why it was so easy to cut her ties to the cast when it came time to write her out.