Super nice project, thanks for sharing, amazing!
The ICM-20948 natively provides the opposite capability: there is an FSYNC input that you can connect to the VSYNC output of the camera (I am guessing the MPU-9250 has it too?). We are currently exploring this for a future version of the sensor. In principle it should work, but it is very unclear from the docs or the sample code from TDK exactly how (they do not implement anything related to FSYNC it seems). Presumably, the DMP will issue a message or set a flag in the header1 or header2 of existing messages each time an FSYNC pulse is received from the camera.
Would that be usable? We just got the new boards back with that VSYNC-FSYNC connection setup and will be testing it soon and writing a driver for it.