Or as an alternative to that, are there small, light, foldable "trollies" onto which one could place panniers (or suitcases) to transport them around?
yes there are, traveller's shops and even some big box stores
should have some..
handles telescope out, to luggage towing height.
Plan on adding a folding luggage trolley to the touring load?
it may be rigged up to make a wider platform on the rear rack
while en-route..
A square figure 8 ring,,that you can clip the pannier hooks onto,
that will then support a padded shoulder strap?
Ortleb bags each have a shoulder strap, but juggling a F & R pair
+ a rack bag and a HB bagis still a PIA.
but there is the old Chinese model of a long pole with a load on either end of it.. Hod carrying the old fashioned way..