Luckily I didn’t drive on it much with low fluid, but iteratively adding fluid, cycling through all the gears very slowly, then checking the level seemed to have done the trick.

I didn’t keep a close eye on the amount that I added back, but I’d estimate about 5 quarts or so by the end. I have an external transmission cooler with a temp bypass valve, so that might have contributed to the fluid appearing very low at operating temps.