I'm working on a project where I disguise my Teensy4 as a mouse. I also rely on Serial communication. The only option I can use under "USB Type" is "Serial + Keyboard + Mouse + Joystick". However when using this, the device shows up with a keyboard icon (understandably).
What I suggest is adding an option for Serial communication and mouse simulation only.

I have tried changing the source code, however it seems that the keyboard and joystick stuff are deeply integrated in the files in hardware/teensy/avr/cores/usb_serial_hid/usb_private.h and so on, so I haven't been able to do it myself.