Would lowering the pre-gain in APCworkbench be a solution? In that way you would still be able to control volume directly via the Up2stream module.
Another solution would be setting a fixed output volume through UART commands. This works for the Pro V3 module, but hopefully also for the mini. You can set the fixed output volume by sending the message ‘VOF:100;’. This would fix the output volume at 100%. Documentation on this suggests that is only binary (1/0) on off setting, but in testing this I have found this not to be the case. In your case you could then set the fixed volume at a level where there is no audible distortion. Adjusting the volume slider will then have no effect on volume. Bonus: the module still sends UART updates on the volume settings, which you could use to set the volume by app on an external device through a MCU. I plan on using this to set the volume on an external ADAU1701 DSP so I can use more digital resolution for the digital volume control (24 vs 16 bits).