I finally found the time to start testing and measuring. I use iPhone 11 Pro and 16 Pro for testing, plus a notebook via USB C with the BRAVO ASIO driver. Lots of unexpected issues that keep me busy to find out what's going on. What is already interesting is that I am not able to perform a THD+N sweep of the Fosi without running into complete nonsense values. Reason seems to be that the High Performance Analyzer mode of the APx gets disturbed by the high amount of out-of-band noise, a known property of the whole CS DAC chip series. Turning the High Performance mode off fixes that but limits THD+N to around -105 or so dB.
@amirm might be able to fix his sweeps by adding his AP switching amplifier measurement filter. And maybe that was the reason he had an 'instrumentation error'.
Back to the bench...