My experience, for what its worth. I've been dealing with Mac BT issues since 2014 with an rMBP. Solved that with a remote IOGear 521 plugged into a wired Apple keyboard port. I also have a 2018 mini with the later Broadcom BT module. Its connected to a Dell Ultrasharp (about 3 feet away) with a built-in usb 3 hub. My mini experience:
Under Mojave:
- Connect the Dell's hub with a usb 2 cable. Use the usb A port on the outside. Eliminates any potential of interference at the mini or display end.
- I use an IOGear GBU521 dongle plugged into one of the Dell display's ports. Plugging the dongle into the keyboard works as well. If you don't have distant ports, use a usb 2 extension cable to get there.
- Run sudo nvram SkipIOBluetoothHostControllerUARTTransport=%01 to turn off the mini's internal BT module. (sudo nvram -d SkipIOBluetoothHostControllerUARTTransport to reset). Sys Prefs>Set-up BT Mouse, Remove the mouse. Restart. Re-pair mouse (may have changed to its default name).
- I use a Magic mouse, performs flawlessly. I use either a wired or wireless Apple keyboard, both operate flawlessly.
- I have nothing plugged into the inner usb 3 type A port. However, with the dongle, using it for a usb 3 external drive or hub is not a problem.
- If I set the mini to use the internal BT module with the above configuration: With no usb 3 drive or hub in the inner type A port, Magic Mouse is ok at best. Its useable but there's visible stutter and lag. With the usb 3 drive or hub in the inner type A port, BT is useless.
- I use 2 Thunderbolt ports for Thunderbolt and 2 for USB-C. One drive is immediately below the mini, the rest are dual Thunderbolt or USB-C enclosures about 3 feet away. Irrespective of configuration or placement, I don't experience any interference from the Thunderbolt ports.
Under Catalina:
This surprised me. With the exact same configuration as above, using both type A ports, the mini's BT is fine. No need for a dongle. However, I did not get along with Catalina (10.15.4 & .5) and returned to Mojave. The mini is primarily a server for whole house media and while disk management has materially improved with Catalina (DU actually works), Apple's continuing focus on security made connections with remote clients more of a process than it was worth.