I agree that it's not nearly as easy as epub merging was. But I think a useful tool can still be had from it for basic splitting.
My vision right now is to offer a list of 'split lines'--the beginning of each file and/or each TOC entry--and let the user click and choose which they want to include. I agree that trying to automatically decide where to split is beyond complex.
Then the program includes only those files (or partial files), scans them for included css & images, writes only those files, manifest and TOC. TOC gets flattened along the way (that way lies madness).
I have that much working in CLI already. (Still need to fix internal URLs, worry about cover image, etc.) I plan to offer & collect metadata using calibre's edit metadata dialog, similar to EpubMerge.
That sounds to me like a useful tool because to do it manually would take a number of steps in Sigil to split/delete files, clean up TOC, plus Modify ePub to remove unused img/css files.
People who are looking for absolutely perfect epubs will still need to hand edit, but I think this will be useful.
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