Even after running just two apps and waiting a very long time to see if they were fully synced, I still couldn't get the notes to stabilise.
That's why I decided to take another drastic step: I deleted my main profile in my Android app and started from scratch with the synchronisation from the Joplin cloud.
After more than 24 hours, no further synchronisation progress was visible in the Android app. However, the synchronisation status has since shown a fairly constant number of around 5500 attachments that have not been downloaded.
To get another reference value, I installed a Joplin app on an iPad for the first time.
After about 24 hours, synchronisation was also complete on the iPad. In contrast to the Android app, all attachments are displayed as downloaded and decrypted on the iPad in synchronisation status.