You can sort of workaround the highlighting limit if you copy text to a note instead:
select start of 'highlight'
move to end of desired 'highlight' (but don't select)
press space bar
The text will be pasted into a note, which you can then edit (add bracketing quotes, etc.) and save. Instead of a highlight 'underline' you'll just have a note indicator.
There's a limit on the number of characters that you can copy in this way (128 or so?), but depending on what you are doing, it might be enough.