I received some advice from more experienced bowyers that the shape wasn't ideal, but that since it was still drawing over weight, I could probably correct it and reduce the draw weight at the same time. In the photo above you can see that most of the bend is near the center of the bow, and the ends are doing little of the bending.
It took another couple of days to carefully reduce the thickness of the taper to the ends, scraping a little at a time, re-stringing, exercising the bow by drawing it 30 times, and then checking the draw weigh and shape at 21"
Here's what it looked like at 16 pounds draw weight, when I decided to quit while I was ahead. Black birch is so stiff that the ends were getting very thin in order to make them flex more, and I just didn't feel comfortable going any farther.
But at its present state it pulls very smoothly, with no feeling of strain at 21" (I've pulled to 22" just for safe measure). The curve (or "tiller") is better and more of the bow is working. I tried it in its unfinished state, and shot 30 arrows into a hay bale, and was surprised at the penetrating power -- it buried the fletching on several aroows. And surprised at its accuracy, too. In fact i was having more fun shooting this small bow, I think, than my old Fred Bear recurve!
It still isn't finished, but I think it's going to work out well. I hope to shoot several hundred arrows after finishing to make sure it is safe and doesn't take too much set in the process, before handing it over. I'll post pics when that's done.