Stewing away happily in our apartment these past couple weeks, the umeshu, umeboshi, and hachimitsu all seem to be doing well. The umeboshi are fully submerged in their own ruby red brine, and it's time to reduce the weight on them by half. The recipes I've been consulting recommend this step once they're under about two centimeters of fluid, and I thought I'd share a photo of what they look like at the moment.
I can find no definitive answer on why the weight should be reduced. My hunch is, based on the tiniest of allusions in a variety of articles and recipes, that the fruit becomes increasingly fragile as the pickling process continues. Reducing the weight by half increases the chance of fully intact plums. Regardless, the plums should remain submerged. This prevents them from being exposed to air, which keeps a yeast mold or film from growing.
Sometime in the next week and half the rainy season should end, and that's when I'll start looking for three consecutive days of good weather for drying these little gems. Fingers crossed it all goes well!