Not sure if this is a simple one, but could the highlighter use a Multiply blend mode or be rendered beneath the PDF linework? Adjusting the opacity helps, but the current highlight dulls the text and lines. Multiply blending would allow the colour to affect the background while keeping the linework sharp and would let us use brighter highlights.
It would also be great to have a point-to-point mode for the highlighter. I often use area takeoffs only as visual references for my linear takeoffs, but they clutter the Takeoffs window and end up buried in grey [No Custom Property] folders because of my sorting settings. A point-to-point highlighter would give me the same visual reference without creating unnecessary takeoff data.