Reading these reviews reminds me that I haven't actually watched the whole final arc in a while. There are a couple of episodes I tend to rewatch quite a bit, but some of these "build-up" episodes I tend to neglect.
On the Breen joining the war... yeah, it's a twist for the sake of a twist. The idea is to raise the stakes, or to give the audience the feeling that the stakes are being raised. But I don't think that was really needed since the stakes are already pretty high, and the Breen really don't add anything of substance.
It actually makes sense that the Dominion would be looking for allies in the quadrant, a bit like Sisko trying to bring the Romulans into the war on the Federation's side, so the idea is good. It's just the choice of the Breen that seems a little odd.