Due to the likely GPU-Compositor interaction, I'm thinking that... (temporarily)
    turning off the Plasma Compositor could improve some desktop stability,
    while attempting to apply a remedy (GPU driver update, configuration fix, etc.)

For Devuan excalibur:
System Settings > Input & Output > Display & Monitor > Compositor
    Compositing: [ ] Enable on startup  (unmark this setting)

(Other thoughts and insights are welcome. Long-term desktop stability is important)

Oh, <sheepishly> I happen to be one of those folks who benefits tremendously from having an LTS kernel.

It's interesting that a newer kernel solved it, and that points to a GPU driver related hang or similar.

Plasma is more demanding than some smaller window manager and makes use of the GPU for acceleration and desktop composition.

Unfortunately Debian and Devuan follow enterprise focused LTS kernels, which though they do backport security fixes do not generally provide fixes for other bugs, or any updated drivers.

I experienced these same stalling symptoms with KDE plasma on X11 recently (freezing for 15-90 seconds), on a Debian 13 VM (after upgrading directly from Debian 12).  The unresponsive UI behavior was occurring every 15 minutes or so.  The mouse worked, and all the background services worked flawlessly.  It was just the KDE Plasma UI.

My best assessment is that a UI timeout was occurring, waiting on some UI resource that was no longer available (or was misconfigured).
I also could not resolve the incessant UI problem.
(Perhaps someone else can provide the proper troubling shooting technique. [--see blackhole's reply below--] ⬅ thank you)

Sadly (or fortunately), my only solution was to do a fresh install of Debian 13 in a new Virtual Machine, and migrate all of my system settings (/home, /etc, /var, /usr/share).

However, three days later, I did this exact same process to successfully migrate this VM away from Debian 13 and onto Devuan 6 (Hurray!).  On the new Devuan VM install (OpenRC), I mounted the previous SYSTEM disk as '/00.PREV_SYSTEM'.

Then by using 'meld' as root, I compared every appropriate folder and file, and migrated the appropriate settings. (Note: you will learn a lot from this.)

sudo apt install meld
sudo apt install dbus-x11 #(this installs dbus-launch)

# dbus-launch meld & # run meld as root
dbus-launch meld /etc /00.PREV_SYSTEM/etc &
dbus-launch meld /var /00.PREV_SYSTEM/var &
dbus-launch meld /usr/share /00.PREV_SYSTEM/usr/share &

I then migrated the 'previous' /home directory by moving the entire content in this precise manner.

# make a full archive copy of the previous home directory
sudo mkdir /home-PREV
sudo cp -a "/00.PREV_SYSTEM/home/." /home-PREV

Check the content in /home-PREV before proceeding to the next step (i.e. ensure the content is there, correctly).

This next step REQUIRES attentive precision. (all on one command line... ONLY)

sudo mv /home /home2; sudo mv /home-PREV /home; sudo shutdown now

When you restart your new Virtual Machine, your system should be properly migrated (for Excalibur to Excalibur, but I migrated Debian Trixie to Devuan Excalibur).

-- I pray that your mileage will NOT vary (and that you will have great success).

P.S. ("in for a dime, in for a dollar" )

You also have to reinstall your Applications and Packages

Get the previous list of installed packages from the 'old' VM.

dpkg --get-selections > ~/package-list__PREVIOUS.txt

Copy the file (or its contents) to the new VM.

On the new VM, get the current list of installed packages.

dpkg --get-selections > ~/package-list__INSTALLATION.txt

Now compare the lists, and ONLY install what is appropriate.

meld ~/package-list__INSTALLATION.txt ~/package-list__PREVIOUS.txt &

P.P.S.

Also, you will need to reinstall any Flatpak applications.

sudo apt install  flatpak  plasma-discover-backend-flatpak
# Use "flatpak list" to view a list of installed applications (and support libraries)
flatpak list

Sometimes my KDE Plasma session freezes and becomes unresponsive, maybe every 1-2 days or so. It starts with everything freezing except my mouse. After some time, windows become responsive again, and I can use them normally. However, my panels and desktop continue to be frozen and cannot be interacted with until I restart. I've tried to look at Plasma's logs, but I'm not sure where they are. ~/.xsession-errors and /var/log/Xorg.0.log aren't timestamped, so I don't know where to look in them, /var/log/sddm.log is empty, and /var/log/runit/sddm seems to only contain a log for the current session. Does anyone know how to fix this, or can help with finding relevant logs?