Instead of one single iCalendar file, perhaps you can write and update individual items in a directory, where each file is a VALENDAR with a VTODO inside? That is something that the existing file backend in SyncEvolution can read and write. The file backend uses file modifications to detect changes, so you would have to make sure not to rewrite files unless they really changed.
That's certainly doable. I'll probably make a single-file or multi-file option choice available as a command-line option.
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