If you dare, you can try temporarily killing the system’s swap (using the swapoff command) and see what happens. With no swap, the standard OOM reaper should trigger within a couple of minutes at most if it’s needed, and it should write an entry to the system log indicating which process it killed.

Note that the process killed is not necessarily the one causing the problem. I haven’t had the OOM trigger on me in many years (I normally run without swap), but the last time it did, it killed my main browser instance (which was holding a large but not increasing amount of memory at the time) rather than the gcc instance that was causing the memory pressure.

Hmmm. Random guess: does your machine have another audio output, possibly via HDMI, that you’re not using? This could be ALSA selecting the wrong device as a default, which would then propagate up through the stack.

Used to be that KDE would let you run other window managers than the default kwin. If that capability still exists, you might just be able to borrow Cosmic’s WM and implant it in your KDE session.

98% of everything should Just Work, although some software may drag in heavyweight dependencies. I’ve used TDE’s versions of konqueror and konsole from inside fluxbox and other lightweight setups, called up thunar from within TDE, etc. At most, you might have some theming issues. The only thing that would be 100% incompatible would be trying to run a wayland-only program from inside an X environment.

Most display managers should be able to handle different window management sessions without issue. If you’re looking at an X environment and really want to start from the WM level, I’d recommend sticking with something like fluxbox, fvwm-crystal, or even enlightenment (which is somewhere between a WM and a very lightweight DE). Avoid anything described as “minimalist”, unless you like the idea of running around adding other software like dmenu and feh to get basic functionality (and like reading documentation).

I vote for “slopesque”, even if it has more letters. It doesn’t hurt that the most common English word that uses the -esque ending is “grotesque”, which this whole phenomenon is.