Calibre has a built-in Open With feature that is very simple to configure and use. A separate list of programs is maintained for each format file type (which can be non ebook file types, e.g. MP4 videos)
You can add a program to a file type Open With list thus:
Right click on the file type, e.g. EPUB, in the Book Details panel and select
Open EPUB with...
Added: You can add & edit Open with programs in the Tag browser:
This will present a list of previously assigned programs,
Click the
Browse computer for program, which will open up a file manager open file dialogue, browse to the program you want - such as Sigil
That's how to configure - now how to make use of it
For frequently used programs I assign a keyboard shortcut to the program:
For occasional use programs I use the
View specific format tool, where there's same the
Open XXXX with... option
This is what I have for EPUB in my Books library
The blue ebook viewer is from a portable 3.48 (I changed its icon to differentiate)
BR
I got fed up with posting this response, in future I'll just post a link