Also, sometimes the omnibus groups books by a different criteria than sequence. I might do it as I do with short story collections-list Omnibus in tags & the individual titles/sequences in the comments.
I'm starting to find that more-and-more Library Management via Calibre relies on comments. Too many situations that don't fit. I've now run into 'sagas' which is a series that contains other series and worse, a 'universe' which is a series that branches. (See Eric Flint's 1632 & follow-ons. I'm sure there are others but that's the one I've been lately untangling so it springs first to mind.) Even using comments I haven't found a decent way to untangle that. Especially when I try to include the short stories in the collections.
And a further problem are series with 'outliers'. In the case of H Beam Piper's Fuzzy Sapiens series he left it incomplete at his death. At least two 'posthumous' books have been written in the series-and the contradict each other. I've taken the one sanctioned by his estates as 'real' and the other as the outlier. (Can't remember the titles offhand-and even if I looked them up I'd need to figure out which is which again since I lost my notes on that one before I figured out how to handle it.)