I think both charectors are supposed to be workaholics, people that lose themselves in their job and basically spurn family dynamics. Miles has engineering and Keiko has botany. Both technical and demanding disciplines.

I think the writers wanted there to be that "we are both focused on our work and not each other" dynamic to their relationship, and wanted Keiko to be frustrated as a workaholic denied, being pushed into both the family role for their daughter and having no plant life to study on DS9, with Miles always forced by the rickety ass station to be absent in workaholic mode. Its a situation that breeds real resentment.

The "become a teacher for the kids" was a sublimation that worked for a while for her on the station, letting her do useful work and watch their daughter, but that of course fell apart.

The other hand of it to is that theirs is one of the few relationships where you can feel the strain of competing needs on Trek. Most relationship issues in star trek are pretty low stakes, even the ones played up like worf/jadeza wedding. Like, it is all going to be wrapped in a tidy bow and we know it. Keiko/O'Brien felt like real people under relationship strain.

Miles and Kieko were both driven people, and Kieko was being stalled out and was frustrated by it. There was no easy answer, just like the real world. Sometimes relationships, even otherwise healthy ones, have issues. Both actors were able to show that well, so they leaned into it.

Unfortunately, it makes for unlikeable people at times, and i think Kieko takes that brunt because she was much more of a side character so she was less fleshed out as a person, and to be blunt, because she was a she. The "shrew" wife is a centuries old trope in literature, and unfortunately the Trek writers left her in that trope too long and too often.