playerctl can list and control (most) media players individually, and would be suitable for writing a script with. In terms of rebinding keys you might be able to set them to run shell commands using global shortcuts in KDE system settings, although I’m not at my computer right now to test it.
I struggled with this for a while, especially since my stream deck is a newer model with knobs and a touchscreen (streamdeck+) so most of the software I tried doesn’t fully support it. Here is some of the software I have tried:
Boatswain -gnome themed, didn’t really like it as it has no knob or touchscreen support (yet), and I don’t think it could run shell commands although I could be mistaken
Stream Controller - another gnome themed app and still doesnt fully support the stream deck plus
Streamdeck-ui - a qt app which can do some simple things like triggering keypresses and running shell commands, I used this for a while
Bitfocus Companion - a cross platform app with a web interface with lots of integrations; I’ve been using this as it fully supports my stream deck, knobs and all, as well as offering a lot of customisation with expressions and variables. I haven’t been able to get the discord integration to work correctly (at least with vesktop), but it works perfectly to control my media player, home assistant and mic. I even got it to display my now playing song and cover art, which I couldn’t figure out on windows with elgato’s official software.
If you’ve tried any of these and it wasn’t working, it could be an issue with udev rules, if these aren’t configured the software cannot interact with the device.
Git Repo, it works on Windows too, just in case you’ve activated it and the bottom corner of your screen feels lonely. You can also set the text to whatever you want