I use Sigil since that is what I have been using for a long time, but Calibre has a similar feature.
If a 'readily identifiable (unique) pattern' exists for where a chapter (target for a file) break exists, just write a S&R to insert the break before the Pattern (remember to restore the pattern if you still use it in the book) NB if the pattern is not Unique, Single step the S&R. 60 chapters: 2 Min tops if it is a clean pattern
In Sigil it is a special tagged <HR /> Yo see a HR rule where the split will happen when you press F6 (split at markers)
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