thank for the answer, and yes, I'm using the series column as shelf information (forgot to mention that in my first posting). Since I additionally use author_sort as shelf, I could most probably could cut the number of shelves in half.
But I fear that even then the waiting time will be intolerable, and thus I thought that by only loading the books on the reader that I previously marked as "to_read", adding to that the accompanying books in the respective series, which is why I came up with the idea of something like a "convex envelope".
But I already got the impression that you only have the metadata of the current book and not of others. Would it be possible to add information to the column data? E.g., add a marker to a series entry?
Regarding startup times: After I ran out of patience I did a hard reset (pinhole) and then waited for another eternity before removing the sd card. And it is half an hour before it starts to show the black screen (the processing itself needs 6-8 hours).