FWIW, Sigil already did that and called it PageEdit which is a separate app that can be used with Calibre's editor via OpenWith and that could be easily interfaced like it has been done in Sigil with its Open Preferred External XHTML setting. That way you can stay with Calibre and still do a bit of WYSIWYG editing.
It has a user interface similar to Sigil's old BookView:
https://github.com/Sigil-Ebook/sigil...s/pageedit.png
https://github.com/Sigil-Ebook/pagee...ain_window.PNG
and can be downloaded from here:
https://github.com/Sigil-Ebook/PageEdit/releases