I have now reset my iPad to 15.7 (the ISPW is fortunately still signed by Apple) and reinstalled DTTG. Synced a database via Dropbox, set the download to “on demand” and downloaded only one file of about 70KB. According to the memory overview DEVONthink needs 375MB of data, which is the total size of the database. But there are no files downloaded …
If I now select another database (with about 30GB), DEVONthink synchronizes (still ON DEMAND) until the iPad memory is full (32GB version only) and then aborts with the message "insufficient memory
After a clean install of iPadOS 15.7 and DTTG 3.5.7, I synced 2 databases with 28.25 GB via Dropbox. The sync settings are clearly set to “on demand” and I did not download a single file.
The memory consumption according to iPad storage info is 10.25 GB.
This makes DEVONThink unusable for me on the iPad, as my total databases are about 200GB.