[chaley]

Originally Posted by Wiggo

I‘m using subgroups for series.

Main series.01 subseries a
main series.02 subseries b

https://manual.calibre-ebook.com/sub_groups.html

I came here to suggest something like this.

I wonder why you don't use something like

Main series.05
Main series.subseries a.01
Main series.subseries a.02
Main series.subseries b.05

This would let you search by main series and sort by series number, or search by subseries and sort by series number. It wouldn't be hard to to build template searches for a subseries independent of the main series ('subseries a' no matter what its main series is), as there are template functions designed to make that separation easy.