There may be another loaf of bread in my future this afternoon. After more research, I think I haven't been kneading the dough enough. The crumb's been a bit dense and gummy each time. Hoping more kneading will improve the results.
That was it! On previous loafs I'd just been kneading the dough in the mixing bowl for a couple of minutes. This time, after mixing it well, I pulled it out of the bowl and kneaded it on the counter for a full 10 minutes. Crumb turned out much more open, soft, and spongey.