Instead of tapping the highlighter (or pen) and then changing the thickness and then changing the color, it would be convenient to have multiple highlighter and pen presets in the toolbar. Settings could be switched with a single tap.

To make room for these extra buttons, the thickness and color items could be collapsed to a drop-down menu for each property with the current thickness and color displayed. (The settings of the active highlighter or pen could be changed via these controls, while a long-tap on the highlighter or pen would still present the existing pop-up menus.)

The attached images show a rough mock-up of a possible arrangement with a highlighter and a pen selected. The Bluetooth icon could indicate the currently selected highlighter or pen (or a simple dot if the Apple Pencil is not used) along with a lighter background (not shown in the images).