I see what you did there 
For something simple like a mono mixdown output over both channels, wireplumber and pipewire have all the tools built-in, so it’s basically a simple matter of dropping a config file. The new audio device renaming feature has that kind of capability, so I feel like plasma could do this job pretty easily.
A couple of links in case the “how” is interesting:
https://pipewire.pages.freedesktop.org/wireplumber/policies/smart_filters.html
I haven’t tested this beyond about 10 seconds, but I’m pretty sure it works: you make a smart filter, with no target set, mono capture and unset channels on the playback stream, and allow it to upmix.
It’ll inject itself transparently before the selected output device, which will cause apps targeting that device to use mono playback, then upmix it and play that through however many channels the device really has.
~/.config/pipewire/pipewire.conf.d/99-mono-downmix.conf
context.modules = [
{ name = libpipewire-module-loopback
args = {
node.name = mono-filter
node.description = "Mono Mixdown Filter"
capture.props = {
audio.position = [ MONO ]
media.class = Audio/Sink
filter.smart = true
filter.smart.name = mono-filter
}
playback.props = {
node.passive = true
node.dont-remix = false
channelmix.upmix = true
channelmix.upmix-method = simple
}
}
}
]
Tada, instant global mono downmix. It effectively makes all your sound cards’ outputs look mono to your apps.
Off the top of my head I can think of like 6 different ways to do this, so if that doesn’t work, don’t be put off, that’s just me ‘doodling’. I’m sure it can work and be something very simple like a single config file, which would be easy for a skilled individual to put in place with a button click or a toggle switch or whatever.
Do I seem to be vaguely on the right track, @kuroki100 ? If I could come up with something that worked for you, do you think it would work for others if it was behind a toggle switch or something? Would it need options or something? Are there guidelines that should be followed for how this sort of thing should work? I appreciate any advice 