Thanks for the answer, Kent. Sorry for the delay, but I have an update
on this.
> 1) Try a different Desktop Environment (e.g xfce4 instead of Gnome, etc)
I tried it with LXQt+xfwm4, but the issue remained. I also tried
updating to the backports 5.16 kernel, but the problem still persists.
The one thing that seems to completely fix the issue is running Plasma
on Wayland. However, Plasma is far from being wayland-ready, especially
given how old Debian's version is, so that's not a viable solution (for
now).
