Yeah that’s basically what you do, but the keyboard’s firmware needs to report itself as a gamepad for it to work, Wooting offers xinput and HID modes, iirc Keychron only does xinput and then Razer I don’t remember. Unfortunately the next best thing was some smaller company that had very limited beta support for such modes, and no other company gives a shit at all.

Alternatively Steam Input would have to add support for keyboards&mice, but the suggestions on Steam forums has been unanswered since 2021, so I highly doubt it would happen any time soon.