The Sort Bookmarks function allows you to select the bookmarks to sort alphabetically by level number. But selecting by level number that is not very useful. Take this example
If you want to sort A's child bookmarks and you choose level 2 then it will sort A's child bookmarks but it will also sort E's child bookmarks which you might not want.
It would be good if you had the option to simply select the parent bookmark you want. There is already a Sort Child Bookmarks tick box. All it needs is the ability to select the parent bookmark as an alternative to selecting by level.
I've spoken with the developer who created the bookmark plug-in and he says that he agrees with your suggestion.
So we've made a Feature Request ticket in our internal system: #5625: FR: Editor Bookmarks Plug-in. Ability to sort only a specific parent's bookmark sub level.
And we will see how we can improve this functionality.
There is also a similar problem with Build Table of Contents. It can only select by level. It would be good if it could also have the ability to generate a TOC only for a specific parent's bookmark sub level.
The Table of Contents is normally intended to cover your whole document! But you can always edit that out once it has been initially generated. I will ask my colleagues to consider the ToC as well - but can not make any promises!
The Build Table of Contents panel has First Bookmark Level to include in TOC and Last Bookmark Level to include in TOC options so it does allow you to choose a subset but, like Sort Bookmarks, the ability to select is likely to inadvertently select more than you want.