Just an idea. Not really highlight: it's more downlight but it gives the attention.
Under settings:
set Paintbrush mode preferences to Shape Recognizer Mode
set Paintbrush Opacity Preferences to 0.7 (or later something else whatever you prefere)
In a "Image-PDF":
Use Tool Paintbrush, strike the stylus over the text you want to hight(down)light. It's now grayed and therefore you have put the attention to it.
Does this help somebody who does not want to underline?