Hey JanHol,
Thank you for using the Apple Support Communities! We understand you have a few questions on how to connect your MIDI controller and other accessories while also being able to charge your iPad. We'd like to help as best as we can.
It sounds like you'll need an adapter, if this is able to be accomplished. Review the information provided in this resource: Charge and connect with the USB-C port on your iPad. We would also suggest reaching out to the manufacturer of your MIDI controller to confirm compatibility and address questions on how-to connect and use it with your iPad.
Best regards.