How far away is the USB Receiver from the Keyboard and Mouse?
Do you have any high speed USB 3 drives or cables near the USB Receiver or Keyboard and Mouse?
see > Resolve Wi-Fi and Bluetooth issues caused by wireless interference - Apple Support
If so, try moving the high speed drives and cables to one side and away from the USB Receiver, Keyboard and Mouse to see if that improves the connection.
The back of the Mac Mini is not the best place. Optionally try putting the USB Receiver on a short USB-C to USB-A adapter, to see if that improves the connection.
https://www.target.com/p/philips-usb-c-to-3-0-usb-a-female-adapter/-/A-79804275
USB-C to USB Adapter - Apple