I'm familiar with Sigil and have used it extensively, but it doesn't covert several fics automatically: you can import a single html file, insert images and chapter breaks and save as epub, or you can build an epub yourself. It will recognize any mistakes in coding you make and correct them, but it's not made for bulk conversions. It's mostly for checking and correcting pre-existing epubs. It's ben a long time since I used eCub, but I think it also can do only one book at a time.

The advantage of calibre is the multiple files conversion (and stripping DRM through use of plugins). That is the one thing I can't give up on, since the time I can set aside for editing and converting my fics is extremely limited. I will check Sanza Desktop, thank you for the link, and if it can do that, I'll be very happy using it.