I have Diaps toolbag on both (the name is different and slight variation in the UI) (I use this a LOT to clean spans, modify deficient (for my use) code. Most common is to simply change a chapter heading to use a H tag so the auto toc builder can do its thing with a click or 2.
Sigil is all about EPUB,

so read the descriptions in the Plugins sub forum.
For Calibre (and Editor), look at the index of plugins.
What you choose depends on how YOU
work , and what you work on.