Options-->Clock-->Clock Source : Auto (will select USB as clock source when USB is active, and COAX when USB is not active and there is a valid COAX signal, otherwise INTernal which is what you want for Phono).
Options-->Clock-->Sample Rate : whatever you want

Thanks for your quick feedback unfortunately none of your answers are satisfying.
@KSTR
Your first suggestion is exactly what I wanted to avoid to turn on a digital device to serve as a „clock“ to play vinyl.
If I change the clock to INT in the settings without any digital source I can‘t adjust the clock because it‘s inactive.

@MC
Your solution only works if I disconnect the USB cable of my Auralic. BTW the same is true for the AD/DA mode.

So obviously my concern is valid and the inactive USB prevents me from setting the sample rate to my desired value for the analog input.
Is this the intended behavior of the ADI-2 Pro?

I didn‘t say that the Auralic is ejected but the streamer is in sleep mode where still some functions are running in the background. However the streamer is not sending music via USB to the ADI-2 but the connection is probably not fully cut off.
My suspicion is that the ADI-2 just believes that it is receiving a signal (noise) and is setting the sample rate to 768 kHz.
Don‘t know if there is a kind of data validation implemented for the USB port.
In any case the sample rate should be never set to the highest value deactivating the DSP.
Maybe MC knows.