I found only a single question on the net asking as you do about "unknown slot argument numbers/types". It is very curt at https://development.qt-project.narkive.com/4qp3VorW/qdbusconnection-connect-to-catch-all-slot

In QDBusConnection there are several connect() methods which check the
signature of the signal against the connected slot.
Since I'd like to implement a scripting possibility around this, I'd need
the possibility to either be able to receive all QDBusMessages (at least
from DBus signals) or a way where the slot just gets all arguments via a
single QList<QVariant> parameter (from QDBusMessage::arguments()).
Is there currently a way to achieve this ?

It is answered very briefly:

Yes. Make your slot take one parameter only: QDBusMessage.

I think this actually means const QDBusMessage &? You should read through Declaring Slots in D-Bus Adaptors (and QDBusMessage Class):

Slots can have one parameter of type const QDBusMessage &, which must appear at the end of the input parameter list, before any output parameters. This parameter, if present, will be initialized with a copy of the current message being processed, which allows the callee to obtain information about the caller, such as its connection name.

And then I see QList<QVariant> QDBusMessage::arguments() const:

Returns the list of arguments that are going to be sent or were received from D-Bus.

Is that QList<QVariant> just what you are looking for? :)

Then (assuming it works!) there is the question of what is in/how to interpret that QList<QVariant>. Per How do I extract the returned data from QDBusMessage in a Qt DBus call? apparently you (may?) need to deal with QDBusVariant:

the variant() methond of QDBusVariant result returns the QDBus Variant as QVariant object .. thus, the call :

const auto &resultArg = result.arguments().at(0).value<QDBusVariant>().variant();

return a QVariant .. that we could easily print in debug or convert to stored value in object

I don't know (not sure whether this is relevant to your messages), but you might also look at https://stackoverflow.com/a/64908626/489865

QDBusArgument arg = args.at(0).value<QDBusArgument>();
QMap<QString, QMap<QString, QVariant>> map;
arg >> map;

Finally, you might also find some help reading through KDE's https://develop.kde.org/docs/use/d-bus/accessing_dbus_interfaces/

That's all I know! Hope this helps? I should be intrigued to hear whether this answers your question!