You make a compelling argument, I'll grant you, but the problem is "Discovery" is still fundamentally a show on a ship, about a ship. It's right there in the name!
So when we're naturally spending a ton of time on the bridge, it's frustrating that over half of the familiar faces doing the important jobs ON that bridge are little more than glorified extras.
Like I said, there's no reason Stamets couldn't also be the chief engineer. I think he's a great character and giving him that job would have just given him more significance in terms of plot as the show moves forward and (presumably, eventually) beyond the spore drive storyline.
Yes, Voyager and Enterprise didn't use their casts very well because they many bland or uninteresting characters. Kim, Chakotay, Mayweather....these are arguably the most dull, least dynamic characters in Trek history.
And TNG did have that problem, too, with the Conn-of-the-week officer, who often times was literally an extra who never got to speak. That wasn't by design, of course, but it was a running change when that show realized they also should probably have a chief engineer instead of a series of one-off co-stars filling the role and after Wil Wheaton left the show.
Now Discovery has that problem times four or five. It's weird that Saru, Burnham and Tilly keep switching off at the science station. A little more thought could have found ways to avoid that problem and give them more specific strengths.
You can come up with compelling characters and then find out where to put them on the bridge. It doesn't have to be the other way around. DSC does have some good characters, I think Saru, Tilly and Stamets are strong characters, but their skillsets overlap to a frustrating degree, while we are still dealing with utterly undeveloped blanks doing 75% of the other vital jobs on the ship. It's just poor planning.
