After resolving some issues introduced when I messed up in the "secret" menu, I finally figured out what I'd done wrong and got my ST16 back to normal. In the process of struggling, I had rerun the latest ST16 firmware update including deleting the data. Once the ST16 was working properly, I rebound the TH and ST16 and recalibrated the accelerometer and gimbal at home. The weather was cooperating, so I took it to my favorite flying site yesterday morning. I recalibrated the compass (camera/gimbal left on to flop around) facing north.
So, this was a perfect opportunity to see how long it takes the TH to be stable in flight after a major change. When I asked a Yuneec customer service rep back in March about the need to wait 12-15 minutes after update or substantial change of venue, he said that it wasn't necessary on the TH. You can fly as soon as the GPS says READY. So, that's what I did. But the bird tended to drift to the right when flying forward and to the left when flying backwards. Also, drifted fore and aft when slewing right and left. Not uncontrollable, just enough to be annoying. After flying through 1 battery and into the next, I did the same test and noticed that the drifting issue was completely gone. So, apparently, all the comments about waiting for the GPS almanac to update are correct.