The correct answer to this exercise will depend on how you think about the tasks being carried out by this filter. Probably the most obvious function to extract is:

\function .enter-transclusion [split[{{]split[}}]get[text]]

{{{ [all[shadows]tag[$:/tags/EditorToolbar]get[description].enter-transclusion[]!sort[]first[]] }}}

An .editor-toolbar-description function replacing the first three steps might plausibly make this filter more understandable, too.