The config file to save is
~/.config/plasma-org.kde.plasma.desktop-appletsrc. You can save multiple versions of this file elsewhere, then copy it back. You will need to restart Plasma after replacing the file. I use the following script, which attempts to kill plasma in increasingly more vicious ways.
# Replace current instance of plasmashell
timeout 5 kquitapp5 plasmashell
pgrep -U $USER -x plasmashell &>/dev/null && pkill -U $USER -x plasmashell
pgrep -U $USER -x plasmashell &>/dev/null && pkill -U $USER -x -9 plasmashell
pgrep -U $USER -x plasmashell &>/dev/null && echo "ERROR: cannot kill plasmashell"
Also note that Plasma takes a minute or so to update the config file, so perhaps save a copy of the file, make your changes, then
diff the two configs until you see the change written.
Also, I find Plasma a bit buggy with multiple monitors. Sometimes it gets confused, and a monitor will revert to the default setting. Basically, you can just set it up again and save the new config. You'll then have duplicate layouts for two variants of the same monitor, but it works fine. (If you check the config file, one will be, e.g.
lastScreen=4, and the other
lastScreen=5. I have no idea why.)