Are you sure you trashed the file named "com.apple.Bluetooth.plist" from the /Library/preferences folder not from the ~/Library/preferences one (in your home folder) ?
Normally there isn't a file with that exact name in the ~/Library/preferences folder or it's from a previous macOS version maybe.
If you trash it and reboot, you'll have a fresh one and you'll have to connect your bluetooth devices again to your Mac.
Anyway, I have the same issue with my Magic Mouse 1 and neither resetting the bluetooth module nor any of the other options nor trashing this file worked.
My mouse keeps connecting/disconnecting when my Mac on Mojave is more than 3 meters away.
It may be a problem with the mouse not Mojave. But I didn't have that problem a couple of months ago with my old 2007 MBP on El Capitan tough...