Have you considered viable alternatives to the drudgery of dusting/cleaning?
Just consider the dust and grime as "weathering" and diorama material that's already partially completed for you.
It'll be a lot easier than trying to scrub off all those layers.
To do this successfully you have to consider the quality of the accumulated dust. If it is tan--consider a desert scene; brown--muddy race tracks or battlefields; light gray or white--winter, of course, etc.,etc.
With a little imagination and a modicum of work the possibilities are endless! Don't waste that 20-year accumulation.
Make it work for you.
"Should we prosper it shall be as is our custom...by Miracle!"