I checked out SD Maid - with a 14-day trial period. After some preliminary work (granting permissions), the first scan showed that 2.4 GB of cache had to be deleted. I had this done and then de-installed the program and deleted the subscription.
The good news: GrapheneOS seems to effectively prevent any "skeletons in the closet". The app has shown that everything is in order.
But I don't really need this app just to clear the cache of all programs. OK, the automated process is 10 times faster than manual. But maybe it could be even smoother without the app. That's the question that remains, and which I hereby put to the community.