Indeed a thorny one
. Totally agree on not wanting to put metadata at row level (which custom columns do) to indicate the number of items in the series because of the maintenance issue. Perhaps if Calibre had the database extended to allow an optional MaxCount or similar field on the series table then you could use that - that way you only have to edit the series tag?
The Missing/Wishlist tag approach helps solve one of the issues, but it doesn't help you record whether the series is "finished" or not. Of course there can always be occasions where years later either the author revisits (or even after death someone else continues it) but these are the exception rather than the norm.
If extending a series tag is not a likely enhancement, I guess maybe another option is another tag like "CompleteSeries" or similar on the first book or something?
I have a slightly similar issue with knowing whether I have all the books of an author or not. Again an attribute at the author level (such as being able to tag authors) would help but not possible currently.
Like I said, open to better ideas with features Calibre has today