Sisko wasn't a Captain because he commanded a Space Station. And no having the Defiant doesn't mean Sisko should be a Captain in rank. He should be addressed in that role as a Captain. But Trek ranking is based primarily off of US Navy ranks. And many, many ships are Captained by Commanders in Rank. Which is why Dax is stated as Captain, during the early parts of the sixth season, but still had the formal rank of Commander.Jessie would comment on the parallels with Sisko as him being the first black Captain and not being allowed to be one (and in fact does) but the thing with Deep Space Nine was that it wasn't supposed to be anything other than a backwater assignment beneath his abilities. That obviously CHANGED with the Defiant and it becoming the focus of the Wormhole but it made sense to start him as a Commander then move him to Captain later.
Burnham not being a captain isn't demeaning. She just has the very Star Trek-esque lesson that it's not a good fit for her character.
As for why the push to give Sisko a promotion was entirely based on fears that the public and the press would see him as lesser to Kirk, Picard and Janeway who all technically had the rank and position of Captain.
It was done for optics.
But the rank was always regardless of the race of the actor, set to be commander. And remember they did audition plenty of actors for the role who weren't darker skinned.