Why does this show do things that feel like season finales midway through the seasons, and then have the actual finales feel like the story's been interrupted in the middle? I guess it has to do with where the novel finales fall, but still, it's an awkward structure.
That's precisely it. They decided that rather than try to shoehorn in a complete novel storyline every season, they'd just take a more holistic view of the arc and allow it to flow out however felt most natural. It makes an odd structure first-run, but the writers' rationale is that we're in the streaming world where shows have long afterlives, so for most of the audience, the difference between a season finale/premiere and any other episode is pretty much erased. Either way, the next episode is immediately available, whether or not people had to wait a week or a year for it originally. Honestly, compared to some other shows, I think it helps with watching it first run, since episodes of The Expanse feel more like
episodes, and not just chapters, or even worse, forty-ish minute chunks of a twelve-hour movie, as happens with some shows. Besides, the fact that the finales don't usually have the most intriguing endings compared to other episodes help with making the hiatus more bearable.
Also, since I'm seeing around the web that a lot of people (including me) couldn't make out Holden and Naomi's last lines in the closing montage, they were:
HOLDEN: You're not staying, are you?
NAOMI: (agreeing) It's time.
That'll teach them to have characters murmur important lines while naked.