I had Bluetooth issues with my new MacMini 2018 from the beginning. The new KB was mostly OK (occasional disconnect, but rarely) and the new Magic Trackpad 2 was mostly OK (occasional disconnect; more frequent than the KB, but not by much). However, my older Magic Mouse was unusable. It was constantly disconnecting and reconnecting, and was really jittery when it was connected. It was worse when an external drive (USB-3) was hooked up and in operation.
I tried all the usual things - securing the batteries, deleting the mouse preferences, etc. Some of those things would work for a few minutes but then the problem would come back. Finally I started thinking about my WiFi router. It's a pretty old Airport Extreme, single band, on 2.4GHz. I've been wanting to upgrade it anyway, so I got a new (non-Apple) dual band router and hooked it up.
End of Bluetooth problems. As long as I'm signed in to the 5GHz network, all devices work flawlessly. If I switch to the new router's 2.4 GHz band, it works OK for a while but soon starts to degrade again.
This is not intuitive. What does the Wifi band have to do with Bluetooth devices? Well, it's above my pay grade, but I do know that USB-3 works on 2.4 GHz, so somehow the Wifi using that band messes up some BT connections. Regardless of why, this solution worked for me. AFAIK most routers are dual band these days so you probably don't even need to buy a new one. Just make sure you have a network on 5GHz and use that.
