I appreciate the input.
I should have been clearer...
I will change the filter, but don’t want to fool with other stuff. The well-under-$100 price is fine for me for the AC checkup.
A vent/duct cleaning is what is a bit pricey, but it’s been almost five years, so I’m going to have it done (by someone) whether there is any observed problem or not.
I have no intention of letting the system fail before I sink a few dollars into maintenance.
More opinions still welcome.