How do I get a list of all of the available dbus properties?

Have you tried using the d-feet tool?

sudo apt-get install d-feet

Try that and see if you make progress.

ref:
man gdbus
man qdbus

In a terminal window run

qdbus | sort -V |                                                      \
sed -u -e 's/\(.*\)/echo =========== \1 ========== ;                   \
gdbus introspect -r  --session  -o \/ --only-properties -d \1/g' |     \
bash

Standard interfaces are listed first so all properties for them are found - the script will halt on the first missing interface DBus address that is listed - ...

(To avoid this halt, preprocess qdbus | sort -V to a file, remove missing interfaces and then sed that file.)

The same can be done for --system instead of --session starting with qdbus --system.

To do just org.freedesktop.DBus.GLib.Const

gdbus introspect -r --session -o / --only-properties -d org.freedesktop.DBus.GLib.Const

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How do I get a list of all of the available dbus properties?