Bjoern, I don't know whether BookManagerv2 is able to "feed" Collection;
I asked a few months ago at
https://phabricator.wikimedia.org/T28448#302254 (cc Raylton), you could
file the request as separate bug. A comment says «Extension:BookManager,
which provides both automatic navigation and automatic print version».
https://phabricator.wikimedia.org/T17073#203184
I doubt anyone in Wikimedia ever really tried this extension yet (cc
Wikisource, Wikibooks). You could check those 3 wikis:
https://wikiapiary.com/wiki/Extension:BookManagerv2 (there's also an OER
wiki of Guatemala it seems).
Again, this is uncharted territory... if the extension gets some real
world usage and feedback outside Wikimedia, its development may get
faster and after some years it might end up being used in
Wikisource/Wikibooks too; don't hope for it to go the other way round,
because it never happens.
Nemo