A couple of years back I made some mods to the filters.
I added small foam insulation strips around the edges of the filter screens.
They had an adhesive side which I pressed into the plastic cover.
These strips block air-flow (w/ dirt) from going around the edges of the filter carrier.
(When I remove the filters, the strips remain on the cover).
So, I'm getting very little dust into the actual HX vanes.
Both indoor units are staying pretty clean.
The post (with pictures) is here somewhere..
Early on, I did use a very soft brush on a handheld vacuum to clean off dust bunnies.
Mostly on the surface of the HX vanes. The trick is not to bent the vanes.
They are
very thin aluminum.