Alright, I think we finally may be onto something. I switched to a high-density foam roller for this next coat, and though it looks kind of eggshelly in the photo, it actually ends up drying to a smoother finish than any brush so far. And it uses less paint, which means no drip/stalactites around the edges, which need to be cut/sanded off.
The rhythm is sort of set now: when I start a new coat, I flip the panel over and wetsand and tack the coat from two sessions prior. That way, the coats go on each side sort of interlaced [I'm painting both sides, and I think it'll be done when the panel builds up a sufficient edge of paint, not support, and there's a pretty clean, non-painterly surface with no discernible front/back.] They're not there yet, but it now seems like they will be.
On the question of posting this kind of log/journal-style info, yeah, it's still kind of boring to me, mixed with a bit of incredulity that really, why would anyone care? But that's fine.
Because one of the things I've found is that these posts almost always draw out some helpful and interesting emails from people who know paint far better than I do. So it's really nice to hear from people, to get advice and feedback, to check my assumptions, and to see what other people are doing.
Part of my decision to paint was to learn what it's really like, to see what paint does, how it behaves. And part of it was definitely to actually see some particular objects in person that I've seen in my mind, and which I haven't really found anywhere else. So it's all pretty good.