That's always been my feeling about Brooks, which makes sense seeing that he got his start as a stage actor. Stewart did as well, but Brooks didn't make the transition as smoothly IMO.
Well, I think, at least, what you're describing is why opinions on Brooks' acting tend to vary a lot. For some people, it feels overdone on camera, as he does tend to retain a bit of the exaggerated style that you need for stage acting.
I also think, that, in the early seasons, he is just bored a lot of the time. The writing for Sisko isn't very good, for the most part, early on.
Once the writers start to really get a grasp on Sisko's character in season 3, and then especially starting in season 4, Brooks really starts to become a strong point of the show imo, partially because his stage-acting style and powerful voice grant him a slightly "larger-than-life" quality that is good for a Trek Captain to have.
Stewart as Picard is a hard act to follow, but, I think, especially in the final 4 seasons, Brooks' work compares favorably to that of his predecessor. If you compare Brooks in In the Pale Moonlight
to Stewart in The Inner Light
, for example, I would say that the performances are of at least comparable quality.
On the whole, I do think DS9 had the best cast, as far as the talent of the actors. The TNG characters are all likable, but they're also relatively bland for the most part, and the roles are less demanding. The actors also tend to do less with what they are given. Stewart and Spiner are the standouts on TNG and, while what Spiner was able to do with Data is certainly wonderful, as far as how charismatic and charming he is while playing a robot, it doesn't really vary or evolve very much.
Nobody on TNG does anything that even remotely resembles what Nana Visitor was able to do with Kira over seven seasons, for example, as far as creating a fully fleshed out character (she does benefit from a lot of very good writing, also, though).