I had the issue - early Magic Mouse (with batteries - not sure if 1 or 2) and a Mac mini 2018. I had a cheap 7-port active USB-3 hub plugged in with my UPS, an external HDD and Wacom tablet attached. Did all the usual - reset PRAM & SMC, reset Bluetooth controller - nothing.
While I was waiting for Apple support I found this thread and unplugged the hub - instantly fixed. Now have my USB devices attached via a passive USB-to multiport adapter that was cheap on Amazon (I bought it for reading SD cards on my iPad as it cost a fraction of the Apple adapter).
Clearly Apple have not done enough susceptibility testing on the Mac mini and what annoys me is that they drag through hours of useless attempts at 'fixing' the issue when they (should) know very well what the issue is from threads like this. As soon as I told the tech I had fixed the problem myself, all he was interested in doing was using my time to gather data on my setup for his logs....
I wish Apple would be more honest with their customers - my Mac mini is still within the return window - not sure I want to risk this occurring again - think it's going back to Apple - those NUC PC's look good - same performance, open architecture and half the price.....