Thanks! From all I've read, including your advice, it seems the trouble could be that the firmware is not updated, however, the DJI Go App does not indicate an update is needed in "overall status". It did a couple weeks ago, and I told it to update, and now it does not show that an update is needed. I wonder if it updated correctly. Perhaps I missed a step.
It appears that on other drones (from what I've read), a person can update manually by downloading the firmware on the SD card and then re-inserting it into the drone and updating until it beeps differently and a green light shows. However, the instructions on the Phantom 3 4K say only to update via the DJI Go App, and you'll be prompted to do so, but I'm not getting prompted. Meanwhile, I'm still getting the IMU error.
It's surprising to me that there's not a way on the app to see what firmware is installed, so I can see, as you've indicated, that it should be v1.2.10. As it is, I don't know what firmware is present, and unless the app prompts me to update, I have no way to update it. Frustrating and seemingly restrictive. I hope there's an easy solution I've overlooking.