Jadzia was the Science Officer of DS9.
Yes. I specifically noted "less popular". They still exist. In the case of DS9, I think it makes sense over say, the Enterprise-D.
The Enterprise-D has a vast suite of science labs with a multitude of science personnel on board specializing in their various fields. Having a general generalist science officer isn't particularly necessary. In contrast, DS9 has essentially nothing in the way of science personnel beyond Jadzia. That makes sense... it's a random backwater space station, and they have a whole-ass planet they could get science specialists from if need be. So yeah, having just a general science person on tap is a good idea.
Personally, among the first season chief engineers (and there was at least one episode with Picard speaking of "one of our chief engineers"), I was rather fond of MacDougal. And I loathed Lynch. The whole reason we didn't have a chief engineer in the first season principal cast is that the Great Bird didn't think we would be spending much time in Engineering. And we did have a regular helmsman for the second through fourth seasons: Acting Ensign Wesley Crusher.
It could have been Argyle, if the actor hadn't just been absolutely awful behind the scenes about it.
I do find it kind of funny that the Enterprise-D cycled through so many engineers at the beginning. I've kind of always thought that (in-universe) they were trying out something different, and rather than having a single Chief Engineer for the ship, it was SO advanced and complex that they had a team of highly specialized "Chief Engineers" working a bit more collaboratively. By S2, it turned out that experiment had largely failed and a true Chief Engineer was needed.
And historically, in the ST milieu, "transporter chief" is a job title for a person (a non-com, specifically a "chief petty officer," once ST admitted that Starfleet even had non-coms; TMOST postulated something to the general effect that everybody in Starfleet was a "qualified astronaut," and therefore an officer) who operates a transporter as a primary duty (as opposed to a chief engineer who could rebuild a transporter on short notice, and do it with people in the pattern buffer). Not specifically the most senior transporter operator.
I do still like the idea that while there are non-coms in Starfleet, it is still an officer-heavy organization and I tend to look at WHAT it means to be an "Officer" and "Enlisted" as being quite different than what those terms would mean and imply today, given Starfleet's not-strictly military organization.
I really think that rank in general is not as rigid in Starfleet was would be in a modern day military, and that Enlisted ranks aren't really what they are in the modern world, but more just "Did not attend Starfleet Academy". For most positions, it really doesn't matter if you are Enlisted or an Officer, only really becoming relevant at the commissioned rank of Commander, which has been presented as a fairly major step in command.
I don't see Starfleet having any issue with a Chief Petty Officer acting as a ships Tactical Officer or some such, or a general department head. If they wanted to pursue a command track, they could get up to an equivalent rank of Lt. Commander (and I do mean "equivalent", at least in general, with potential caveats), but would have to go through some sort of process to become commissioned and move on full Commander.
As for the Transporter Chief as a role, I think... yes, mostly that. But I don't think a ship would have multiple "Transporter Chiefs", I DO think that the role IS a senior role and O'Brian was in charge of the transporters on the Enterprise, and capable of at least some amount of maintenance on them.