I think my main problem with Keiko is she's filling the role of "TV wife." This was a common trope in old shows, in particular: The wife has me on a diet. The wife's out of town, so let's have a Poker Night. Wife's upset that I was out too late with the boys. Wife has me sleeping on the couch again.
Now Keiko does have a little more going on. She's unhappy because she doesn't have a fulfilling job since they left the Enterprise. Miles wants to help, albeit a bit clumsily. This all makes sense. Keiko ends up opening a school. But of course she does. All women love children, right? She can be the school marm, out on the frontier! Keiko is a trained scientist. No science needed on DS9? No knock on teaching. I'm a teacher, but that's not Keiko's speciality.
When the school fizzles, Keiko finally gets her shot as a botanist. Yay! Good for her. When she does come home for visits, however, it's more like she's in the way.
In a show with strong female characters like Kira and Dax, Keiko feels like she falls into a lazy the wife trope because nobody knew quite what to do with her. I think they genuinely tried and she does have moments, but to me she never really felt like a believable fleshed out character.