I have found that AC systems are genuinely dirty & gross; First an air conditioner sucks up dirt, dust & hair to store it in the ductwork; You need to get ductwork cleaning every 5-7 years, however look up pictures periodically of old ductwork.
It will make your skin crawl how gross it looks.
The air filter also catches debris from getting into your system. It can only do so much. The inner workings still need a fantastic scrubbing from time to time. There is more than just dust that is a problem as well. The cooling coil in the system also is a wet venue. Where there is wet, mold forms. A lot of people don’t even realize they are blowing air over the mold spores inside their system. It is smart to at least once a year have the unit learn to avoid mold & mildew growth inside of it. Another area is the condensate drain. So much water is flowing here consistently. It is simple for algae to back up & clog the drain. Have you ever seen an air conditioner leaking down the sides, spit out water or be in a puddle of water? This is all due to a condensate drain that is clogged with algae. It is an simple fix, just muck it out. It is genuinely gross however it makes a world of difference. None of the AC cleaning jobs are hard, just icky. If you are willing to get your hands dirty you only need to call an air conditioner company for repair once a year.