My system didn't have /etc/kernel/ directory, but creating one and following your instructions has solved my issues. No more flickering or incorrect resolution upon reboot.

In T9640#182907, @STiAT wrote:

I'd be interested in if (in plasma) ALT-F2 + "plasmashell --replace" actually remedys the issue for the current session or if you get those issues even replacing the plasma sesssion, since that would be kwin issues then (kwin_x11 --replace)

Thanks for response.
Running "plasmashell --replace" seems to have no positive impact, however running "kwin_x11 --replace", whilst not setting everything to a normal state, at least enables more resolutions on a second screen and so I'm able to manually set it to 1920x1080 without having to reboot or mess with (un)pluging of cables or turning monitors on and off. Do you have any suggestions on how to narrow down potential kwin issues?
And to reiterate, the majority of issues aren't present with the LTS kernel.