The mouse as a controller is an amazing fluke, If asked to design the perfect controller for a first person game no one would build a mouse. But that thing designed to point at stuff on the screen turns out to be unmatched at looking around in 3d space. I think it is the huge range of precise motion it affords, hundreds of millimeters vs the single digits afforded by most other controllers.

So for the couch... I don't know... perhaps a vr hand controller would work. I Have never tried one, so I do not know if they are as precise as a mouse.

The keyboard is the weak point in the mouse and keyboard setup, it is nice because it has a lot of buttons, but there is no subtlety to it, just a bunch of binary buttons. does any one make a one handed thumbstick controller? Something like the wii nunchuck controller. A long time ago microsoft made the strategic commander, a sort of captive mouse with a lot of buttons for your left hand. I never had one but it looked like a good idea. I have a spacemouse(if you squint it is sort of the same thing) but have never been able to map the controls in a way that felt satisfying.