How do I convert a 3-axis quaternion to a 1-axis x-y axis plane?

Thanks again.

Gyros can be calibrated upon program start

So I can have a maintenance procedure in production that does this and stores the results in flash. Right?

The MPU-6050 has not been manufactured for several years

Unfortunately the factory in China that makes our boards has a specific list of parts we can use and the 6050 is the only attitude sensor on the list. It is just a toy though.