Linux

You could do that at the firmware level, with QMK or ZMK macros (or, presumably, whatever other firmware). It might be a long one, but launching an application or the like could just be typing the combination that runs it. I haven't used KDE, but something like super, then type the name, then enter, should work.

Having said that, a quick look at keyd proposed by the other replier does seem like it has more than enough capability, and if you have one setup you want to use it for and not move the keyboard between computers, it very well might be the better choice for you.

So you want use a second hardware keyboard to trigger custom key events or sequences, right?

I don't use Wayland but keyd works on the evdev/uinput level so I think it should work regardless of the display server used. To use it for the secondary keyboard only you'd list only that device in the [ids] section of the config.

kmonad or https://github.com/snyball/hawck?