having a usb keyboard & usb mouse (as most of us desktop users probably do) i don’t have a sys-usb appvm at all (since it’s greyed out during installation). During testing before switching to Qubes i managed to lock myself out by installing sys-usb.
This probably won’t happen when i install usb-keyboard as described here:

or does it?
Are there any other (security) benefits with creating/using qvm.usb-keyboard? Or can i just ignore this?

I also have a desktop with Qubes installed. But I have purchased PS/2 converters for mouse and my keyboard is not usb. Anyways those measures were due to me being paranoid (I think).
If you have a usb keyboard, you should be well aware of it’s USB controller and don’t assign it to sys-usb otherwise you will be locked out whatever method you use to create sys-usb.
In case of mouse there is not so much problem in assigning its controller to sys-usb. but allow it to pass to dom0.
Main point is know about your usb controllers with qvm-pci
There is definitely security benefit of sys-usb but in your case scenerio, I think you should or have to avoid it.