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Henning - the following tools can help you to separate "Book records" from "Series records"
Virtual libraries
Quickview
The calibre:// URL scheme
View Manager plugin
You can edit metadata in the Book List using the F2 and Tab keys. The Virtual Library and View Manager tools will allow you to create a specific book list view for editing the "Series records" in place. That would allow you to configure the Metadata Editor to be optimal for "Book records".
FWIW: I do all single record metadata editing in the book list.
BR