As of today, Goodnotes offers two ways of highlighting. One can either select the text manually, and click on 'Highlight' or use the Highlighter tool with a stylus/finger and freehand it.

I propose a method that is highly common and immensely popular with other simple pdf editing apps. Clicking on a Highlighter tool, which then enables the user to drag across text to highlight precisely. Clicking on a word with the tool engaged highlights only the word; dragging across words highlights the words, sentences, paragraphs, etc. The difference between freestyle/freehand method and this is that this tool enables one to selectively highlight only text faster and accurately without all the stroke-related messes that freehand highlighting causes.

It's a relatively simple feature to implement from a developer point of view. Simply utilise the text layer of the PDF to find and highlight text that is passed over by a finger/stylus.