The mouse is working since 5.0.0.43 or some minor versions before. With Bluetooth my Logitech MX Master 3 didn’t work but an very old Logitech mouse works. So it depends. Cable connected mouses and keyboards works as well (with an USB Hub powered by the Phone in between). The Landscape mode is currently 5.0.0.77 not usable wit an mouse independent if its connected with an cable or Bluetooth. See [C2] Mouse Axis are interchanged in Landscape-Mode
The Xperia 10 III supports an external monitor with Android. It’s not scaled but an copy of the screen is displayed. With 5.0.0.77 currently both of my phones reboots ~10 seconds after the cable is connected to the docking station. See 10III crashes with beep when plugged into docking station