Are these AC ducts beyond cleaning?
Those ducts are not that dirty. The bottom picture shows what looks like dirt in the bottom of the duct but the sides and top are still shiny. The duct in the top picture is the dirtiest. The insides of these ducts are made of rigid insulation , not sheet metal. First and the cheapest possible solution is to place charcoal briquets (not the self start kind) any where you can inside the ducts, for a couple weeks. If there is a spot where you could place a netted bag full of charcoal in a duct, do this also. Last idea, Home Depot and Lowes both sell a cut to size charcoal combination filter and charcoal filter called "the web sbsorber", and costs about $10.00, that can be mounted in a filter rack or "made to fit" elsewhere near the blower. The green filter, filters the air and the charcoal takes out the smells. I have been using these for years since my wife has a nose that detects odors form 1000 feet. I replace the filter every 3 months and run my fan 24/7. This works for me. BY the way, We moved into an apartment 20 years ago while our house was being built and I did the charcoal in ducts and netted bags in the furnace housing to get rid of the musty smells.