[un_pogaz]

Yes, but no.

Did you know how many editing software for EPUB there is?
I did some research. A deep search. And so I can tell you with certainty that there are only two.
Sigil and Calibre's.
And that's all.

Oh, HTML and XML IDEs exist (VS Studio, <oXygen/>) and other export to this format (LibreOffice, Atlantis Word, Jutoh)
There is well oXygen which manages a little better to identify the EPUBs as ZIP archives, and thus opens them as simple folders (it just avoids decompacting the book each time)

But none directly support the EPUB.
By that, I mean a management of the OPF, the meta-data, the TOC, the files inside the EPUB or any other specificity of this format.

Sigil and Calibre are the only ones, and in addition, by simple but complete interface, and in a transparent way.

HTML IDEs are powerful to do all from A to Z, but you really do all from A to Z (every file, every line, every word, and a single modification and you should check and probably edit any reference to that)
I do not hesitate to get my hands on the code, change what already exists but create from 0 and manage the OPF and TOC is monstrous.

If Sigil became a specialized IDE for the EPUB, he would not only take a vacant position, but also very much in demand.

Well, I'm on fire and I know it would be a huge job.
I don't know C++ that's why I only suggest this approach (otherwise I would already be coding for Sigil).
I'm not asking to reinvented the wheel, I'm just asking to install it on this great and rare car that is Sigil.