The frustrating thing is, this should be easier than it is.
There are lots of example of people using an ATMega chip to talk to USB as HID. I have found circuits which I have built. I have found code and sketches which I have compiled. And for me, nothing has worked.
There seems to be a secret bit of magic missing.
There also seems to be a shortage of people that understand this stuff. Rather than having input like "the resistor value for your pull-up should be 1K larger" or "your zener should be closer to your connector". I see comments like you should be using a "different processor" or "different Arduino" or just buy something off the shelf. This is particularly frustrating when you then watch a YouTube video doing just what you want to do with a chip and a few discreet components on a breadboard. It would be nice if those videos include a links to schematics and source code.