On Windows, there is a method of highlighting in Microsoft Word, where the highlighter button can be clicked on with no text selected, so that the mouse cursor can then highlight text merely by selecting it. Such simple highlighting functionality would work wonders in the OneNote app for Android.
Add softer highlighter colors. Such colors appear on the Kindle app. The neon colors are quite strong for any viewer.
This should be beyond obvious. Clicking on the highlighter and then hoping to be able to draw a straight enough line to highlight the desired text is ridiculous. This increases the time to highlight and slows down the actual reading and mental intake due to the frustration. As far as anybody with arthritis or certain disabilities it would seem to make its use difficult or impossible. IIRC - it took Adobe many years to add simple highlighting to their reader but it works well now that it is there. And to add irony... typing this message I selected a range of text and it was highlighted in yellow no less. That's how easy it should be. :)