I'll try Sigil again, it's been about 10 months since I used it, (and uninstalled it).
Yes, a sort of WYSIWYG editor, because it seems at a glance to be a small step from the calibre reader's current capabilites - but I am probably quite mistaken here!

Ah yes! Because I only use ePub, I'd overlooked that other formats would have to be supported.
What's "Open with PI"?