iPad mini 6 used as music server/ battery life

Last point first…

Your iPad will suffer no harm in being connected to external power for extended periods - whether in use or not. This usage model is common in Point of Sale and Kiosk applications. The iPad will optimally charge your iPad battery, modifying its charging strategy as appropriate.

More information about iPad’s battery management can be found here:

About iPad and iPhone Charge Management Feature – Apple Support

As for your iPad draining its battery while connected a USB-C cable, this is perhaps more unusual - and may be indicative of a defective cable. Certified USB-C cables incorporate active micro-electronics within one or both of the USB-C connectors.

It is perhaps unclear how your making the audio connection between your iPad mini6 and your audio system. If using an analogue audio connection via a 3.5mm audio jack, there are good quality adapters (e.g., Satechi) that provide both a USB Power Delivery port and a 3.5mm audio jack and associated DAC. Similarly, there are many USB-C hubs that similarly provide multiple ports/interfaces of various types, including analogue 3.5mm audio.

Using an appropriate adapter/hub resolves the issue of keeping the iPad battery charged and healthy - while providing connection to other ancillary devices.

You’re very welcome. Hopefully now armed with actionable information, you’ll be able to refine your setup.