You’re right, the database cache seems like its for you since you have a lot of query data coming in that changes - that can really take the load off mysql when done effectively.
My question is: what is the source of the changes for the database queries? For a blog, it would be the point where you create a new post or update an old one. If it is something that is maybe randomly taking a quote of the day or something, you could create a cron script that deletes the folder with that cache data in there.
It all depends on the type of data and the point where it changes - thinking about it that way you wouldn’t need to compare the cached data and the updated query.
What kind of data are you pulling and how is it updated or changed? I (or someone else) might be able to point out an ideal solution with that data.