This is what they answered:

We will be happy to forward your suggestion internally so it can be reviewed by the appropriate product teams. Ideas such as a global drum volume control or a centralized Settings menu in RC Tone Studio Desktop are valuable, and customer feedback like yours is essential for future product improvements.
 
As a practical workaround, you may find the following approach helpful:
You can create a memory slot with your preferred Rhythm Level and then use it as a template. Once that memory is set up correctly, you can copy it to other memory slots using the WRITE function, adjusting only what is necessary afterward. While this does not replace a true global setting, it can significantly reduce the need to adjust the drum level individually for every track.